The general consensus is that the game is excellent, although seemingly (and predictably) a bit rushed. Apparently, sometimes the game moves so fast, it's frustratingly difficult to react to obstacles in time. The odd camera system is no help to this matter, being that it frequently swings behind objects in the path, blocking the player's view of the character entirely. Most suggestive of the game's rushed release, though, are several glitches where it's reportedly possible to get your character stuck, forcing the player to reset. There is certainly the possibility that these glitches may be fixed before the game's US release, although there is no official word.
Well now that we all know what's wrong with the game, what makes it so good? The big change is apparently in the way you enter levels (Zones). There are two main RPG-style "cities", where you can walk around talking to the locals and getting hints on where to go next. The object is to find the entrance to the appropriate Zone...that's right, you have to find 'em. (There have been some problems with this among importers who aren't thoroughly experienced in the Japanese language, as they can't read the "villagers"' hints...but you can't really complain, this version wasn't intended for English audiences.)
Once inside a Zone, the gameplay is Sonic all the way. High speeds, countless branching paths, Ring-grabbing, Badnik-busting, loop-spinning, take-it-all-the-way goodness...in 3D! Aside from the traditional branching paths (creating the possibility for hours of exploring goodness, just as all the 2D Sonic's provided), the game takes a cue from Mario 64 by offering the player a number of other "goals" in each level. The first is usually (but not always) to simply reach the end of the level...that's the only one necessary to advance. But going back in opens up further goal options, such as completing a Zone with atleast 50 Rings, among others (almost like Sonic R). By completing these tasks, you are awarded with a Sonic Token...the complete number of Tokens available in the game is unknown, but it's probably safe to assume that collecting them all is awarded with something special (and hopefully more than a guest appearance by Metal Sonic giving you 100 lives...).
Please note that I don't actually own the game...alas, my willpower is being pushed to levels beyond what I knew I could endure as I painfully await Sonic Adventure's US release. If any of you kids out there own the game and can further detail the information given here (without spoilers!), please hit me! Just for the record, I get about 40 hits a day, but I haven't checked in about six months...so you'll be reaching out and touching over 40 people! Isn't that exciting? ...well, isn't it?!
Forget it. Here's a bunch of people in Sonic costumes touting the game on its day of release. It's severely disturbing that Knuckles lacks the white stripe on his chest, but man...isn't that Yuji Naka costume realistic?! I'd swear it was really him if I didn't know better!