I just bought and beat Sonic CD on XBLA. And realtalk, I'm still not a fan of the game. I still find the level design messy. The color palette is way too loud; it's just to show off what the Sega CD could do. And the architecture of the stages themselves was cluttered, frequently to the point of being really frustrating. Wacky Workbench is the worst offender, but Collision Chaos really got at my nerves too. Especially the boss.
While I think the peel out is a fun mechanic, I'm kinda sad spin dash isn't as useful. And what a tease to make it look like the regular spin dash but still have it act like it did in CD. Rolling in general seemed to be less effective, actually.
The time travel stuff is pretty meh to me. I do like that you can get rid of every enemy in the stage, but aside from that, time travel was just there.
The Special Stages are tech demo material too, but I didn't mind them too much. They're tough, but they're made that way. It's not like you're fighting the game's mechanics to succeed at it. Though if I had a nickel for every time I would just miss and jump under a UFO, I'd be a rich man. The new quick saves helped a lot.
Bosses are gimmicky as well, really. None of them aside from the final work like the Genesis ones where it's 8 hits and he's dead. You always have to get rid of some obstacle in the way first. And again, I didn't really mind them. Except for Collision Chaos. The pinball stuff just never worked the way it should. And Metal Sonic is really difficult.
Sonic CD is not a terrible game. I certainly don't regret spending the $5 it costs. It's just not for me, and I think it's a bit overhyped. I did not like some of the directions it took (to illustrate, I liked Adventures of Sonic the Hedgehog way more than SatAM. I know, I'm weird), and perhaps I'm too spoiled by the later Genesis games. It might be a toss up with Sonic 1, but yeah, 2 and 3&K I think are far superior.
Oh yeah, the soundtracks. There are some that I like better on the JP soundtrack, and there are others that are better on US. And even still, some like Stardust Speedway or the Special Stage just depend on my current mood. Both have their weird parts though. But that's what makes some of them fun, kinda like Super Sonic Racing. I might give the US soundtrack a tiny edge though. Quartz Quadrant and the speed shoes jingle are infinitely better on US imo.