I'm a big fan of the XXVc setting, and I've often toyed with the idea of rebuilding the system into d20/d20 Modern.
In fact, a few years back, I played in an online version of it that was a very simplistic d20 refit. (I believe I still have the quickie PDF that the GM made, if anyone's interested PM me. Be warned though, it's all text and not very well "molded" into the setting.)
Beyond that, let me say the setting is what REALLY shines in this RPG. I think the attachment of Buck Rogers and his cast to the RPG is what really killed it. Too many members of my gaming group had negative feelings toward the Gil Gerard TV series to want to play XXVc.
But the setting? Rocketships, Genies, terraforming, mass-driver guns, mobile cities, space elevators, energy barons, pirates, secret mining operations -- man, that was some good stuff! Lots to exploit as a GM.