To make them last, do not keep them loaded when not being used. If they are 20 or 30 units, then back them off a few rounds, it will put less pressure on the feed lips.
You will find the followers give trouble for the feeding over time. The plastic used tends to be a little 'grabby' in nature, and a touch softer than Pmags and Troy mags. What can happen is the inside will get scored, and then the material grabs at the follower just enough to slow the springs' ability to feed the next round quickly.
When it eventually happens, you will see a consistent FTF at a certain round count: on all my thermolds that have issues, which are 10 round units, rounds 5-7 are a stick point. On all of them. If you load 10, it feeds the first 3-4 give or take, and then you will have to rack, and push the mag up. Fire a single shot, and do it again. Then, the last 3-5 rounds will usually have no issue feeding as normal. This is due to the plastics used, and is the nature of the beast. There is no way to smooth it out, and oiling it will not do the trick on a 100% basis either. In fact, the oil will hold grit, and compound the issue over time.
Just be patient, and use what you have. When the magazine drought ends, USGI mags are $10 at Brownells, and they work perfectly. Pmags run about $13-16, so that is not bad either. Just be patient, and watch for stock to increase fairly soon.

This is what concerns me about any wonder-plastic used to make magazines,see pic.

Attached Images

glock mag cracking-1.jpg (39.0 KB, 247 views)

A couple drops of super glue, and those will be good to go!!!!!

Never had a good experience with Thermold mags, though they may have been the 'cheaper' version without the glass reinforced polymer. However, I think you've answered your own question, though you seem to be somewhat contradictory. If 'expensive' doesn't necessarily equal 'better', 'cheaper' far too often does mean exactly that.

I stake my life on Magpul Pmags. I have 30-rounders which have never failed me in any way. I don't know what you consider to be "expensive", but no price is too high when my ***** is on the line. Before the Newton incident, I paid about $14 for a Pmag which is about $5-$7 more than a Thermold. If you think that's too expensive, I'm at a loss for what to say. Unless you're in dire need of 'em now, I'd say to hold off and get either USGI, Pmag or Troy Battlemags.

I have 8 thermold 20's that use for varmint hunting. My Sports are not my self defense/life on the line guns. I have a bunch of USGI's 20's and 30's, Stoner 10's, TangoDown 30's. When I first bought my Sport, I stretched our budget to the limit for a couple of months and wanted more than one mag. I havent had a problem with my 20's, I read a lot of reviews, tons of them. I know they will wear out faster than my other mags. I load them to -2 without a single problem and will use them until I throw them away but at $2.99 I have more than got my money out of them, its a year later and I am still using them.
Where I live my life is not on the line, yes self defense and home security are an issue but not a problem. We havent had a break in or burglary or murder in our little town that I can remember and I am in my mid 50's. It helps to be in rural america and off the beaten path.
I have other guns that I would grab for a quick response if needed. Guns that have phenomenal close range stopping power. Inexpensive doesnt always mean unserviceable nor does expensive always mean quality. I would hate to live in a position where I am having to "stake my life." They work great for prairie dog hunting off of the bipod where 30's are a little unhandy. Also mine arent the Canadian made that used the cheaper material. But they work great for what I bought them for.

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Grover Oathkeeper

I must say that statement "Time-tested quality and reliability. That's Thermold" is a bunch of ****! I purchased five 20 rounders back at the beginning of the year (2013) and wasn't able to try them until the beginning of Mar. due to the fact that I was waiting on my upper & lower to come in. The day for testing finally arrived I loaded all 5 mags. with 5 rds. of assorted ammo .223 & 5.56 and as I placed the mags. upon the carpeted ledge of the shooting lane, the one mag. just decided to spit out 4 rds. all over the floor of the shooting lane! ...
As I proceeded to recover all loose rds. I set that mag. aside to try another one, this time as I inserted the mag into the mag well 2 rds. decided to dislodge from the mag.! I tried another mag. and this one seemed to be okay until I tried to chamber a rd., as I pulled the charging handle the bolt stripped not 1 but 2 rds. causing a jam! Needless to say at this point in time I was highly aggravated so as not to loose any more precious range time I loaded up some of my 30 rounders (NHMTG Aluminum) and proceeded to zero in my red-dot. So as far as I'm concerned these Thermold mags. are JUNK! Yes I did return them for replacement (they would do a refund) so I'll see how this next batch doe's!
Will post updated info. after I receive the replacement mags. and field test them.

I have 3 as they were the only pre-ban poly mag out there, however now that may not make a difference as NY has gone to 7 round max.

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This Signature is too Short!

I use nothing but GI 30rd mag's with my AR. Have been using the same 6 mags for close to 30 years! Can't get much more reliable than that. Have no opinion on Thermold mags, never used one.

I have 6 thermold 20's that I bought right after I started this thread. I got them for $2.99 each with free shipping. I figured if they were junk they were junk. I also ordered 4 30 round Thermolds at the same time. All American made. The 30's never failed me but basically I kept them loaded and didnt use them other than the original operation check. I sold them relatively quickly for more than I paid. I use the fool out of the 20's. There for awhile I was switching them back and forth between 2 MSR's both Sports. Still (knock on wood) are going strong and perfectly serviceable. The bulk of my Mags are aluminum or steel and they work great but in my case I cannot really bad mouth Thermolds, mine have done very well!

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Grover Oathkeeper

I have one of the older 20rd Thermolds that I use for my .300 BLK. As for the few 30rd ones i did have. I sold them to the l33t guys who thought spending $40 a piece for a Thermold 30rd was worth it.

My 20 has never let me down, but I personally wouldnt own any more Thermold 30rd mags......You just cant beat a 30rd Pmag.

I have a mixture of 30's. Mil-surp w/ Magpul followers, PMag, TangoDowns and am happy with all of them. $40 for a Thermold 30 round mag....LOL...The Thermold 30's are flimsy but the 20's are a good dependable throw away mag.

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Grover Oathkeeper

I had two Thermold 20 rounders that I used for duty carry in the car as the rifle fit better in the case with a 20 round magazine in place. After a few weeks the feed lips spread and the Colt would not feed the first few rounds reliably. The magazines were only loaded to 18 rounds. Rounds would actually pop out the top when the magazine was removed from the rifle. Both magazines did this.

I would not rely on these magazines for serious use.

Thermolds were not and are not designed for loaded storage. If you keep them downloaded by a round or two and if you don't store them loaded they will usually work just fine. Interestingly, when I first started using an AR, that was the same advice given for the GI aluminum mags (20 round) that we were given...download and don't keep them loaded for a long period of time. Having said that, I've got 30 round Thermolds that I have been using and abusing since the early 1990s that have been just as reliable as my GI 30 round mags.

I had a 30 round Thermold mag that I bought used, US made. Had it just to have it.
Loaded it and it split down the back about 1.5 inches. I sent it in for warranty in the early 1990s and didn't hear back from them for about two months. Called in and they told me that because it was older they would not replace it at no charge. I asked them to send it back and they wanted to charge me for shipping. I thought I could put some epoxy on it to give it a try. I work at a well known place that makes gun accessories, we don't treat our customers that way.

I dont think that I would carry my thermolds in a mag pouch, Mine stay loaded 1 or 2 down in either a range bag or waterproof ammo can so they dont get jostled around much, My metal mags are the ones that get jostled the most but I also treat my rifle and mags as if they were made of china...LOL

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Grover Oathkeeper

I've got ten of them that I bought just before the '94 ban went into effect. I never really used them that much. They don't have the anti-tilt followers. They seemed to function okay, but I noted recently that empty mags don't seem to want to drop freely consistently from my new RRA. I don't see any defects or damage, so it may just be them and the gun. I don't know that I'd use them for anything more than training. I've had good service with P-Mags and respectable service from the current crop of GI mags as long as they're take care of. In regard to current Thermold purchases, they're probably good range magazines if the price is right.

I seem to recall when I bought them (pre-Internet) from an out-of-state distributor that they were allegedly designed to be loaded and forgotten. I never did that with any magazine. I think I was also told that they were the same as Israeli military issue (or my memory is getting cloudy). Anyway, for as little as I've used them over the past 20 years, they've worked. The springs are still strong.