I really do not like this as much as the first one. First off, they lost the jank. The cutscenes are still pretty amusing, if not as wacky (tbf Sonic meeting the president is fucking hilarious). The story is more serious, which is both an improvement and to its detriment. And that's kind of my criticism of everything in this game: lots of "improvements" have been put in place. Some are great, like the lightspeed dash being instant or the Knuckles/Rouge stages being genuinely great this time. However;
Sonic/Shadow stages are more linear and strait line speed-focused, which may work for some but skipping vast parts of the level was something that made the original incredibly exciting. The Chao garden is far better integrated, but realising the keys and animals are the only worthwhile collectable in stages leads to my rapid decline in any interest of them once I get bored of Chaos.
Worst of all is the character change. I would be completely fine with the way in which you move from character to character per stage, I'd go as far as to say it IS an improvement from the previous games 6 character stories that are all just the same game again but varyingly worse than Sonic's. This is only in theory. In practice, Tails and Eggman exist. The mere presence of their stages in this game, just knowing once I beat a genuinely fun Sonic/Knuckles level, I'll be back to the horrendously slow prison of the mech...
Fuck Tails and Eggman for ruining this otherwise solid sequel.