Better than you think, and much better than previous Sonic letdowns. For more, look here.
My review;
Upsides;
First off, the game has an AWESOME script, I found myself thanking Bioware over and over and over. For example near the beginning Sonic said, "Ah Green Hill zone, good times." And tails replies, "By good times you mean Eggman turning all our animal friends into robots?" "Yep, Good times." was Sonic's reply, now tell me honestly, would you see Sonic Team write that, 99.999% say no, you wouldn't. The gameplay is really good, for an RPG with Sonic, it is credited less than it should be. Also, the skills and choice of what you want to level up each time is really nice. There are 10 chapters in the game, and some longer than others. Sadly, the game is NOT very challenging until the very end... (UNLESS YOU HAVE CREAM DARNIT...) The first 3 pre-bosses are hard, and it is important to remember to use your skills and items often. Chao are really a good concept for that game, having another hand held style chao garden is what I was far from looking forward to. Chao help you in your battles with upgrade such as, 'HP slightly regains for each party member each turn.' There are 40 chao in all, and it is unlikely you'll get them all before you win it the first time. After winning it the first time, you get to play it over again with all your level ups kept. Speaking of skills, a tip I must give is you really should be careful not to just get each skill but to level the good ones up to the max. The game was fascinating introducing another character, this time, affecting Knuckles. What I thought was fun about this idea was that first you gotta fight against her, but then she joins you. As does Eggman join you. Eggman has an MAJOR ego just so you know... The music in this game wasn't bad at all, the sound effects were good, but they were a tad loud, for example, I'd have my DS on half volume, and you could hear the sound of getting rings VERY clearly. The graphics in this game were... well, they varied, the battle graphics personal looked rather chibi, and a bit to cute, not detailed enough. However, running around the maps seemed okay. And the cut-scenes where comic style, very nice. The story line of the game is awesome, because it's fresh, and doesn't have too much of a questionable ending. The credits were also fresh like the rest of the game, the credits were in speech bubble form, because tails was telling Sonic about who made 'them' in the game. (I read it all and I recognized a few people... I = nerd.) I also liked that you could save anywhere you wanted to. (I will ALWAYS love that.)
Downside;
A downside however, was the fact that the game felt rather short in the long run, and lacked some explaining here and there. I'd say it took me less than 24 hours to win, but I didn't win it in a week. I spread it out to see if it was memorable by loading it and seeing where you are. Another down side was that you often ran into unnecessary battles where you would get damaged before bosses and have to use items to heal yourself so you wouldn't die. Sometimes the case was you could get away from the unneeded battles by fleeing instead of getting damaged, but there was always the case of failing to do so, or just getting tired of it. (I did this several times to avoid battles and most likely over 30 times...) Another downside is that whilst inside battle, it was annoying that an enemy would do a PP (or Pow/special) move, right before you did, and killed you, and even if you didn't die, you'd have to follow the pattern of your own skill after they're skill... (