I bought my 638 about 2 1/2 years ago. I do not care for the internal lock but I have learned to live with it. IMO, the internal lock system reeks of something thought up by some landshark lawyer(s) and adopted by a bunch of executives in a boardroom but then again, just my opinion.
I feed my 638 a diet of practice 158g lead semi wadcutter or flat nose copper jacketed that both loads get around 800-825 fps range. My carry loads are the lighter, faster 125-135g jacketed +P hollow points. I have NEVER had an IL failure after many hundreds of rounds through the gun. I did not care for the factory boot grip so, I replaced them with the Houge one piece wraparound grip and have never looked back. I do not have large hands but the boot grip would not allow space for the pinky finger. The Houges are larger but I live with it and I find the combo quite manageable.
The 638 firing +Ps can be a handful of recoil slap so, a larger aftermarket grip may be the cure.
I practice on half size torso targets at the 7-10 yard distance and the shot placement and groups are almost always more than acceptable in the way of being accurate. I do like the almost concealed hammer and the DA/SA capability.
The 638 is however limited in the way of barrel length, capacity, and caliber but once these limitations are understood and one trains accordingly, the 638 is quite a defensive package if one takes advantage of the fine choices of defensive ammo available.
Since I bought my 638 it has become my constant companion and has seen many trips to the Colorado high country and will continue to do so guarding against any trail hopping, peak bagging bad guy that may be out there. I just drop it a cargo pants pocket or a fanny pack and I never know it is there.
Once you take posession of the gun, practice is the key and lots of it. Get some speedloaders and some dummy rounds and practice your drills at home. These guns are far more durable than they appear but do understand the 638's limitations.
In case anyone wonders, I do not pack a 638 for defense against black bears or mountain lions as it would be almost useless. In my many years of hiking/climbing/peak bagging, these critters have never been an issue. The odds of meeting them are very small. Most black bears just head for the nearest tree and anyone that I know that has had the rare instance to see a big cat, the cat appears for a brief second or two and is gone with no harm done.
Good luck and good practice with your new toy.
Ordered a new S&W 638 last week and I'm picking it up tomorrow. I did a lot of reading before deciding on this model. Last night I was reading on another form about all the horror stories of the ILIF on these guns 
I also read where it would be a fun day in court if you disabled this lock and had a shootout. There are a lot of mixed reviews on this ILIF system. I guess I'm just paranoid now 
But, what do I know 
