I beat this game on an iPhone when I needed games to play on my commute to work a few years back. Just like I thought it would, to me it feels like an improvement over the original. The soundtrack is better, the flow is better, the finale (which took me quite some time on that touch interface) is better, etc. Aside from the Chemical Plant Zone boss where you keep falling through the floor most of the difficulty is fair and can be overcome through trial and error and learning. And compared to the underwhelming end of the previous game, this game's end stretch felt quite challenging and rewarding. If I could think of one issue I did feel like I could skim through some of the levels way easier than some of the original Sonic levels which means I may have missed more stuff by not taking as much time, and it also means some levels might be a bit too easy to get through, but that's more towards the beginning. But that also means that the game does a better job of emphasizing Sonic's fast flow.

So overall I think Sonic 2 builds well upon what the first game did.