Hi Cserv;
I've got a bunch of promag 20's and 30's. They all work, with the proviso that the bolt doesn't always lock open on the last shot as they come from the factory. The fix is to very carefully remove a little material from the follower so that the tab that engages the bolt catch will go up "just enough". Once they're fixed, they're fine.
30 rounders, whether Ruger or Promag all have a quirk, which is that when fully loaded they can be hard to insert in the action if the bolt is forward. Its endemic to the design, so it is what it is. And if you do insert a fully loaded 30 on a closed bolt, expect to dent the cartridge shoulder. Some people freak out at this, but I just shoot 'em and have never had any problems. Dents pop right out on firing IME. YMMV so if dented cases bug you stick with the 20 bangers.
Best,
Grumpy