I got some Pro-mags a couple months ago. At first, they had the problem with the follower coming up too far, which is easily fixed by heating the follower and widening it. The original method is to use a cup of boiling water and a wine cork, but I used a heat gun & a 20 ga. hull filled with lead & crimped, as I don't own a wine cork (and a beer can won't fit!).
This fixed the problem of them coming up too far, but they still wouldn't feed well at all. Eventually I figured out that the follower was tilting front-to-back in the body and causing the jam. I used my heat gun and stretched the legs of the follower out, and now they feed 100% after about 5 fills each (about 200 rnds total....) of various high & low brass ammo.
Bottom line is, if you don't mind fiddling around to get a reliable mag, the Pro-Mag will save you some money, but if you want out-of-the-box reliability, you'll have to spend $30 more ea. I figure the couple hours I spent messing around with them is well worth the $120 I saved.
Rumor has it that Mike Davis (the genius behind the MD Arms 20 rnd drums for those who don't know) is working on plans to make box mags that have the internal steel support like the factory mags.