I actually have (3) 35 round mags that I use in Steel Challenge and not had any issues with them. The only complaint I have with them is there is no window to see how many rounds are left.
The newer plinker tactical mags are fine. Maybe a bit tight on the spring at first but they will work out for you. Trust me make yourself a loading tool out of a metal bbq skewer, it helps a lot. That plastic loading tool they sell leaves a lot to be desired.
The salem 6 mags and the pro mags are much more problematic.
I have one of their older 35rd models...its ok. The thing is way too heavy, even when unloaded and it seems to have problems with the first 5 or so rounds when fully loaded.
In the end, I found it to be rather gimmicky, not really worth the $$. I'll stick to factory magazines.