Gex is one of the closest things to a system seller the 3DO ever had, really.
Oh, and yes, US reviewers did give the 3DO version high scores. I think the later ports (PSX, Saturn, PC) scored a bit lower though, but of course, they were later; the 3DO version came out a good nine months ahead of the first port I believe. (Unlike, say, Rayman, which actually released on Playstation before the Jaguar version was out.)
On the note of Gex's release date, I find it odd how both Wikipedia and Gamefaqs (and Mobygames) all list the 3DO version as having released in November 1994 on the 3DO, despite the box saying "1995" on it and the retrospective by a staff member here
http://games.greggman.com/game/gex/ clearly saying that the game launched for 3DO in March 1995. Um, I think the sites are all wrong there. They are probably right about the Playstation version launching in December '95/Jan. '96 though.