Sonic Rush is a slick blend of the old and the new school. At its base is the tried-and-true side-scrolling Sonic gameplay. Attached on to this base are a variety of 3D and dual-screen features that manage to add to the gameplay without losing anything. What do we end up with? A cohesive whole.
The gameplay, barring a few new editions, is the same as the first Sonic game you first played a zillion years ago. The biggest new edition, of course, is a little feature that's only possible on the DS right now: gameplay on both screens. All the stages feature segments that take place on the top or bottom screens, and sometimes will slip you back and forth between both with a quickness. It can be a little disorienting at first, but you'll get used to it fairly quickly. The actual stages are expansive and feature a good variety of alternate routes to take you through to the end.
Sonic Rush uses a mix of 2D and 3D graphics, almost in the same vein as Viewtiful Joe. Sonic is pretty clearly a fully three dimensional model in some instances, but the stages skew two dimensional. Many of the stages have 3D elements, such as the rideable paws that appear on the Egypt-themed stage, and the boss stages are 2.5D. For example, the Egypt-themed stage features Sonic vs a giant dung beetle on a round platform. You can rotate all the way around the platform as well as make the usual jump and duck movements. It isn't technically 3D gameplay, but it works well and is quite enjoyable.
The biggest addition, other than the dual screens, is the Tension meter. You build meter by doing tricks on rails and killing enemies. The rail tricks are wonderfully remniscient of Jet Set Radio Future in that they take only one key to activate a series of tricks. It's simple and fun, and painfully easy to do. The Tension meter allows Sonic to "boost," which essentially means running even faster than usual with limited invincibility. This is an absolute must, as Sonic Rush tends to place enemies directly in your path after you hit a ramp or go through a loop. Boosting will push you through them without harm, not boosting will result in a bunch of lost rings. Easy choice, isn't it? The Tension meter has a number of levels and is actually pretty easy to fill, anyway. When in doubt, boost.
The Tension meter has another function, as well. Scattered throughout a few of the stages are gateways to Chaos Emeralds. To access them, you have to hang on them, then boost until you're going fast enough to open a portal above the gateway and shoot through. Once you're in, the gameplay changes to touchpad only. You use the stylus to direct Sonic through a long half-pipe and make him collection enough rings to earn an Emerald. These half-pipes are filled with rings, enemies, and bouncers that knock you high into the air and leave you ripe for doing tricks.
There's a couple of playable characters this time. No Tails (please, contain your disappointment), but you get Sonic, obviously, and Blaze the Cat, a newcomer from another realm. She makes an appearance at the end of the first stage and is unlocked for solo play after that. This functionally doubles the length of Sonic Rush, which is always helpful. The story progression works about how you'd figure, where beating an entire stage results in a quick story segment that also ever so happens to move you on to the next stage. Each stage is replayable, as well, which facilitates the collection of Chaos Emeralds and lets you increase your scores and grades on the different indvidual Acts.
Sonic Rush is a pretty full package. If I had any complaints, it'd be that not being able to save between individual acts would be the biggest of them all. The occasional bottomless pit out of nowhere would also be pretty high on that list, but those turned out to be pretty rare. The stages are huge, well-detailed, and genuinely fun to poke around in. As far as side-scrollers go, it's high quality. It'd be nice to have more extra content, but what we have here should be enough to keep you sated for a good while. It's difficult, but not so difficult that you'll snap your DS in half while you play. Have fun with it, it's worth it. This is the best Sonic game in ages.