From what I've learned over at ar15.com, it would seem that the Orlites and Thermolds are FAR better than cheap aftermarket mags, but they're just not quite as good as USGI's. I just bought a Colt AR15 earlier this week, and I already have 4 Thermolds and have 5 USGI's on the way. The Thermolds fit very tight into the Colt, and actually seem to make the bolt harder to pull back. I've heard that this usually goes away after a few uses though. Don't know much about Orlites, but the Israeli's know their weaponry so they can't be too bad.
One common complaint about the Thermolds though, is that under the stress of a combat type situation, the feed lips have been known to crack. However, this seems to have been more of a problem with the Canadian manufactured Thermolds which supposedly use a lower strength plastic than the US made ones. The Canadian Thermolds are identified by the maple leaf on the lower portion of the mag.
USGI's have their flaws too though. Some need new followers to function, they loose their finish fast, and if dented they become unusable. Thermolds can't dent, loose their finish, or need new followers.
Just do like I am and try them all for yourself and see which works better for you.