Enough information about Sonic X-Treme has come out that we have a pretty clear picture of what the game was going to be like. And, man, it probably wasn't going to be very good.
http://www.youtube.com/watch?v=XAv-gNaBn_w
http://www.youtube.com/watch?v=aZb7LE6G5Gg
http://www.youtube.com/watch?v=wzvS_beXtXk
http://www.youtube.com/watch?v=zq_iPxQsbm0
A lot of people like to think it would have been, but I'm a firm believer that they come to that conclusion exclusively because of the old "you always want what you can't have" saying.
Maybe it wouldn't have been awful, but it's definitely not the sort of game you would have sent out as your answer to Crash Bandicoot and Mario 64. It barely even looks like a game sometimes, and that's from a reportedly 70% complete build when Chris Senn and Ofer Alon secretly took over the project and wanted to make it in to a PC game after the Saturn version was canned. And while we're at it, wrap your mind around
that: A Sonic game exclusive to the PC.
From what I remember, Senn wonders why Sega gave him the cold shoulder on that concept. Really? The flagship Sega Saturn Sonic game becoming a PC-only title? You can't imagine why Sega would turn you down?