Played and beaten the same day myself. I'd actually meant to throw my two cents in much earlier, but, well..... stuff.
Spoken as a longtime fan of the Genesis game, I found this to be a very enjoyable remake. Aside from retaining the core level designs of the original, they've created brand new sections for each stage. These are in the form of extensions of an original's stage (like, say, having to marble-toss more blocks to clear a path in Toyland), newly integrated obstacles (such as strategically maneuvering through a thicket of evil trees in the Enchanted Forest), and challenge-survival portions meant to satisfy those who want to go for the harder-to-reach collectables. (Keeping afloat amongst a prolonged rendition of the floating leaves part in the Enchanted Forest.) They rarely feel out of place when stacked against the pacing of the original game, and plenty of them are really quite clever and innovative. My favorite addition is the part in Toyland where you needed to hit switches to reverse the gravitational polarity. In a new key area, there are two switches lined in a vertical row with a platform and pit underneath them -- and you need to puzzle out which one of the two switches you should hit first in order to successfully make the platform appear beneath you rather than above. (Well, easier to show than to tell.) Oh, and the tense, sweat-inducing section at the end where you need to ascend crumbling cliffs up to Mizrabel's tower before the final battle? Epic.
Graphics are neat. The 2.5D look makes the game seem more vast in environment than its admittedly linear structure lets on. I love that every enemy has a 'defeated' animation whenever you hit them.
Downsides? The boss battles are relatively easy affairs. They weren't exactly difficult in the original either, but given the keen attention they'd given to enhancing the rest of the game, it would have been nice if the same was done for the bosses. Except for Mizrabel herself, which was one hell of an amazing battle; among one of the best pattern-based bosses I've encountered in a platformer.
And then the newly composed soundtrack. Regrettably, I've found myself digging Kirkhope's score less and less as I got further into the game. The Enchanted Forest and Toyland themes were terrific arrangements, but then the score goes off on its own thing starting with Storm. The Storm theme got completely replaced, and not for the better. Most of the rest is generic Disney-esque (and yes, I'm gonna say it for the one time that it's truly apt) "Mickey Mousey" fantasy fare. The biggest blow was the re-composed boss battle themes. (I say themes plural, since the new theme has varying arrangements per battle.) It's frustrating because it sounds like Kirkhope uses the barest hint of the original theme, but embellishes it into something far less interesting and memorable. Future playthroughs will undoubtedly be played with the original Genesis score turned on.