D20 Silver Age Sentinels
We played a D20 "Silver Age Sentinels" game for almost a year, but that was a few years ago, right after it came out. I'd put it somewhere between Champions/Hero System and GURPS Supers/Powers in terms of complexity. Note also that it was already an adaptation of a Tri-Stat game, similar to the anime-based D20 "Big Eyes, Small Mouth."
We had a good Game Master and group, but the rules were indeed complex. As a reference to superhero gaming, D20"SAS" is more generic and logical than the old Marvel or DCU games (if not as scalable in power levels), and hardcore fans will still find good ideas in it. We played a superhero/cyberpunk campaign that eventually folded because some of the players moved away, although rules issues were an ongoing challenge.
My group has since then switched to D20 "Mutants & Masterminds" 2nd Ed. with much success. Sure, there are always rules that are broken or that don't make sense, in terms of realism, consistency, or in replicating the universe(s) of comic books, but that tends to happen more often in D&D3.5/D20 and superhero games where numerous powers must be simulated. Overall, I've been happy with "M&M"2e's streamlined take on D20 (a precursor to True 20) and how it captures the mood of four-color comic heroes.