Sonic 4 Episode 2 is a game I'm completely inconsistent with. I played it for half an hour and thought it wasn't worth playing through, played it again from beginning to end and hated it, and then played it through again and honestly thought it was on par with the Genesis games.
In comparison to Episode 1 it's definitely better. The physics are improved and the graphics no longer look like New Super Mario Bros. but worse. A lot of people complained that classic Sonic was absent from the first game, and took it way overboard, but I kinda do agree that modern looks really unnatural here. Thankfully there's a mod for that anyway if it bothers you that much.
The game controls a lot better than Ep1 and it's at least satisfying in this game, but still not quite as solid as in the classic games or even Generations. Sonic feels like he lacks weight on the ground and momentum building isn't quite right, sometimes requiring homing attack spamming just to reach your max speed on the ground.
The level themes are all blatantly reused concepts with the exception of White Park which while I can compare it to some levels it doesn't outright copy anything. I hate the fact that a game selling itself as Sonic 4 is almost nothing but recycled levels and enemies, but the levels themselves have so many new concepts that it's still pretty fresh as a whole, and the likenesses of Episode 2's levels to classic levels isn't as offensive as in Ep1... with the exception of Sky Fortress which is nearly identical.
The game's visuals are absolutely gorgeous. I'd say Sonic 4 Ep2 is the most scenic game in the series. It's incredibly polished visually. I can't say the same about the soundtrack, though. While I initially really hated it, I realized on subsequent playthroughs with Super Sonic available I never used him for just one reason; I didn't want to stop the level music. While I do actually really love the compositions themselves, the instrumentation is horrid. They clearly went for a Genesis style tinny-ness to the synths but it just sounds awful on some tracks, specifically Sky Fortress Act II and the Death Egg's theme, two songs which I would actually really like if they used better synths.
Oh yeah it's also really short. This game has five zones in total and 25 levels in all if you count the boss levels and special stages. I think with a casual runthrough of both this and Sonic 2, Sonic 2 would still be longer, but I consider Sonic 2 to be fairly short for a Sonic game I dunno. Completing the game without all of the Chaos Emeralds yields a message to go collect them, but there's really no reason to, because it does nothing to the ending. Little Planet is still stuck inside the Death Egg while Sonic and Tails feel good about themselves in their escape pods. This could be thought of as a lead-in to Episode 3, but as of now Sega has no plans for one. One of the developers has said that they might make one if Ep2 sold well. Did it? I dunno but I certainly don't want the Sonic 4 series to end like this.
The game was actually short enough that I turned to achievement hunting to elongate it. Which was odd as this was after I'd already had my fill of this game and dismissed it as another bad Sonic entry. I actually WANTED to come back to collect achievements and actually found a lot of fun with the game, both in gathering the achievements and just playing the game itself. However I will say that the achievements are absolutely pointless. Certain gameplay elements like the Red Rings and presence and abundance of rings before boss encounters were clearly just implemented for the game's achievements on release, but getting said achievements has no bearing on the actual game whatsoever. Collecting all the Red Rings or getting the last hit on every boss as Super Sonic, none of this affects the game at all. It doesn't even have a list of the achievements in game so really it's questionable why they're even there other than to make this short game slightly longer.
All in all though I did find a lot of fun with this. The game's length as well as reused settings are the biggest problems, but they're pretty well made up for with a beautiful presentation, pretty enjoyable music and most of all a lot of good level gimmicks and physical designs. Is it as good as the classics? Eh, probably not, if only because it's not completely orginal, but I had just about as much fun with it anyway.