Mutants & Masterminds is the 800 lb gorilla.
I personally like what I've seen of 1st Ed over 2nd Ed. But as with so many things, the people that stay with the older version are in the minority.
Silverage Sentinels had some mixed reviews. Some folks really liked it, others didn't. Before it had a chance to get really established, M&M showed up and blew it out of the water. To be honest though, most of the Guardians of Order d20 products weren't really well received. M&M really managed to nail what a lot of people wanted, and with 2nd edition they addressed a common complaint of "it's still to close to D&D".
Don't forget there was also Deeds not Words which enjoyed brief popularity, as well as Vigilance: Absolute Power from Mystic Eye Games.
Deeds not Words has been dead for a while, but the people that took over the rights to it have actually continued working on the project. It's all taking place here:
http://games.groups.yahoo.com/group/phoenixprojectrpg/
Of course the thing many people are going to complain about is how close the game(s) are to the core rules as you see in D&D. Back when it was all getting started, that wasn't seen as a bad thing. Nowadays? Different story I think.
Afterall, just look at the success of M&M. None of the other d20 supers games have held on.