It looks like it has both 2.5D sections and rails sections.
I am, of course, skeptical. Even in 2.5D they seem to be flipping the camera around in strange ways (though that could just be because it's a demo video - the rooted camera in the final shot seems particularly odd), and what they show of gameplay, as mentioned, looks more like Rivals than Sonic 2.
Hurdles Sonic Team has to surmount on the path to making this game not suck:
- Letting the camera "get out of the way" and focus on what the player needs to see, not cinematic swoops (unlike ... 3D Sonic)
- Making the level design vibrant and interesting (unlike Rivals), with no gotcha hazards and no punishment for going fast (unlike the Advances)
- Keeping this werewolf gameplay something Sonicy in more than name (unlike SA2), yet interesting and different enough to justify replaying the levels (unlike Heroes)
The first one should be trivial in 2.5D/rails if they can resist the impulse to do flashy nonsense. The latter two are significantly more difficult. But if nothing else, Sonic Team is at least working from a solid base this time - we're not in a situation like Heroes where the fundamental game design was flawed.
As for behind-the-scenes speculation. After Sonic06 came out, there were interviews with Sonic Team members in which they were practically despondent. They pretty clearly recognized that they had delivered a turd, and with the combined impact to morale of that and Naka's leaving, there was talk about even losing the Sonic Team name. I think this is the resulting impact: that design responsibilities have been passed away from Iizuka and Nakamura and co on to Yojiro Ogawa, the Secret Rings designer, whose heritage this game inherits. Since this is multiplatform, on Wii/360 according to TSS and almost certainly on PS3 too, it's pretty clear that SEGA has taken the "traditional 3D" option off the table for now.
So basically, the "mainline" Sonic series finally got so bad that it hit them in the pocketbook, and this is the result. My guess is that it'll almost certainly turn out better, but whether it's good enough to genuinely qualify as "good" aside from the Sonic title remains to be seen. It's a trite prediction to make, but my guess is that if this is targeted for Christmas 2009 there is a much greater chance of seeing a good game than Christmas 2008.