A few days late, but that's cool.
The G&P mags blow. You can do the Spring mod as shown in one of the issues of The NAM, but that doesn't always work. The problem is, they double stack and then jam up in the mag. Lubing them works, but then they still jam.
Breaking them in? Well... I'm not spending $25 on a mag, and have to have countless jams and misfeeds because it has to break in.
I have two of them, both work as long as I load less than 60 rounds in there. A midcap that only works with 58 or so bbs? That's bull.
The majority of my mags are star, I have two G&P, and a few marui and CA standards. The CA work, but if you want something that will never fail (as long as you treat them well), listen to Vince. The Marui mags are great.
If you want a good midcap, the MAG midcaps are pretty decent, I just got 4 of them, and they didn't jam or misfeed once.