I have it and am playing it currently.
Right now I am pretty much I think at the second time you fight Ix. And my characters are under level 20. But I got through. What I don't like is sometimes the pow moves. For certain moves it gets really tough (espcially when there are alot of them or they go fast!) I do use the Chao equipment. Some of them are worth zip some are very good to have. (Regeneration of Health or PP) Better chances for Ambushing opponents, better chances to hit the opponent (Luck). The leveling system is different for each character, some are slower some are faster to go up.
For Nocturnes Fire, Ice, Earth, and Lightning are super effective. So I use Fire Chao on knuckles, and the Ice Ring on Shade. I use Omega's Flamethrower alot, Chaos Rift for Shadow is pretty good for fast battles, but don't work on Event Battles. But takes a chunk of your pp if underleveled. Like me where most of my characters are at 18 or 19 possibly 20.
For N'rrgals: Tip: DEFEAT THEM QUICKLY!!! They hurt you they get their health back, worse than the Nocturnes doing so.
For Voxai: Not bad of enemies, but poisoning effects something (But I have loads of antidote)
For Kron: It's been long since I fought them...
For Zoah: I hated them because they damage you hard and they have a bunch of shield breakers
For Gizoids: I hate them, but are easy except for the Gizoid Prefects (That was difficult, 1st time I fought the blue one, I damaged 1 hp each time) ANd the other Gizoids where Tails had to find these 4 special Gizoids that were deactive to save Knuckles. Before that, the fight with one was....
Characters:
Sonic: I always use Axe Kick at times, Blue Bomber and Fastball usually.
Tails: Good supporter
Cream: I don't use much
Big: Great defense
Omega: Counter-free moves
Knuckles and Shade: What can I say?
Shadow: Helpful POW moves.
Rouge: I use sometimes
Eggman: Bombardment ftw!
Amy: FINALLY A GAME WHERE SHE IS USEFUL!!! Besides SonicAdv3
(Did I get all)
Stages:
The one that annoyed me was Metropolis and possibly some others.
POW Moves: Great to use, just must watch PP consumption
I rarely use Defend option
Items: I use a bunch, some I don't.
... Final Score: 7.6