I've mostly got Brownell's mags and they work fine - not a bad deal at under $12 each. The USGI and the NHMTG mags I've got also work fine, as well as both P-mags in my kit.
Thinking back now, the only mag that ever gave me any trouble was a refinished USGI 20 rounder that I got at a gun show during the dark days of the Clinton AWB . . . it was worn out and I was taken in by the nice refinish. Caveat emptor!
You can find a lot of information and opinions at http://www.ar15.com/forums/f_3/17_Magazines.html
Some FAQ information is dated, but overall it's a good starting reference.
One thing - I highly recommend
the LULA loader/unloader accessory! It makes loading easy, and if you want to, you can unload a 30 round mag in a matter of seconds.