The closest to praise I can give this is that it's not Sonic 4.

This game isn't a horrid unplayable glitch-ridden mess like 06 or Rise of Lyric but it sure is another Sonic game that leaves you feeling 'eh.' Superstars is an incredibly weak follow-up to Mania and if there's one thing it does have in common with Sonic 4, it's that it's clearly being marketed as something it was never given the budget to be. (Also please for the love of god don't pay $60 for this. It's worth half that price at best.)

With regards to actual level design... also 'eh'. The levels can be fun to play through but in classic Sonic fashion there's an awful lot of automation or unfun gimmicks thrown in here. Despite initially being said to feel close to Mania, there's been some changes made to Sonic that are entirely unnecessary - he's a bit less responsive now, and for whatever reason the spindash has been nerfed into the ground. (because everyone loved that in Forces!)

The two other notably mixed things here are the music and boss fights. Jun Senoue makes some very nice music... when he isn't trying to imitate the genesis with a dying cat. His music (and some of the others) contrast pretty hard with each other, and when there's 18 composers working on your game it's not hard to imagine why.

Boss fights are generally unmemorable, and the ones that are memorable are only memorable because they're the extremely tedious ones where you have to sit around for a minute and wait for the boss to attack before you can do anything.

TL;DR - Superstars is a half-decent 2D sonic game but it will be forever doomed to live in Mania's shadow (i wouldn't really put it above 3&K either to be honest). Buy it on sale or something if you really wanna I guess. 6/10