Good gameplay and graphics, unfortunate choice of music.
The gameplay is fairly similar to the classic Sonic sidescrollers, although Sonic Team has added a few other abilities to Sonic's repretoire, such as the ability to charge up reserve energy and release it, giving Sonic an instant burst of speed, and of course Sonic will often be grinding around in his usual post-Sonic Adventure 2 fashion. Honestly, I'd really prefer a complete return to classic Sonic gameplay (i.e. "New Sonic the Hedgehog"). All the new stuff just seems like unnecessary clutter to me in terms of gameplay. You should be able to play a Sonic game with only a D-pad and two buttons.
The graphics are very good, but they lose some points with me because of their use of cell shading on the character's sprites- it doesn't fit in very seamlessly with the rest of the game, and reminds me of the worst ripoff Sonic title ever made, Sonic Shuffle.
And that leaves us with the music and SFX- by far the worst part of this game. Ever since the release of the Dreamcast, Sonic Team has been utterly obsessed with giving Sonic a "cool" image in terms of what they seem to think are modern teenage gamers' tastes, and the effects have been far-reaching and overwhelmingly detrimental. This is quite noticable in Sonic Rush, because this game has some of the worst Sonic music I've ever heard (right up there with Sonic Adventure 2's "running around at the speed of sound" track). It's completely uncharacteristic of all but the most recent Sonic games, and is full of "hip" vocals and screaming guitar rips that really make me want to vomit. If that weren't bad enough, Dr. Robotnik (or should I say Dr. Eggman?) is on the trendy bandwagon as well, and can be heard saying things like "get ready to be schooled" in the middle of a boss fight.
It's long past time Sonic Team went back to Sonic's roots and gave Sonic games a more authentic feel, and the first place to start is in the music. I'd also highly recommend completely ditching all dialog :) This game is good, but enjoying the audio experience in a game is very important to me, and this game fails miserably in that department. Maybe next time, Sonic Team!