This is a really great game! Usually, you expect a kart racing game to mediocre, since most don't attempt to come anywhere near the polish or detail of Nintendo's Mario Kart. And that's especially true on a Nintendo system, because in the face of the overwhelming sales advantage Mario Kart has, most developers don't even really try to compete and instead save their best efforts for the versions selling on other consoles, where they are more likely to sell copies. This game, however, is a very pleasant departure from the rule, and gets almost everything right.
First, let's address the elephant in the room: Mario Kart. The Wii U doesn't have a Mario Kart version yet so I can't compare the two, but I find this game much more fun than the Wii version of Mario Kart was. I've also payed the more recent Mario Kart 7 on the Nintendo 3DS, and I thought the Kart / Boat / Plane mechanic in this Sonic game was much more refined than in Mario Kart 7. In Mario Kart 7, it didn't feel dramatically different to handle a boat or a glider than it did a kart, the use of them was somewhat minimal and it seemed like a relatively small evolution of the game. In this game though, the Kart / Boat / Plane system is put to good use and changes the feel of the game dramatically. Boats and Planes handle much differently, and the levels are designed to take advantage of this. They sometimes even have different courses per lap, so you may race the first lap as a car and find yourself flying the second lap. And it's a lot of fun to jump a huge wave in boat or do barrel rolls to dodge asteroids and obstacles with the plane.
And the next thing you notice about this game is how good the controls are. They really feel tight... not loose or unrefined like a lot of kart racing clones. And they differ from racer to racer based on handling characteristics, just as they should. In fact, I personally felt a real difference between speed, acceleration, and control of various racers, which I found refreshing. Not all kart games can make racers feel individual, but this one succeeds.
Something else very pleasant about this game are the level designs and graphics. The level designs are in beautiful HD, and the level design feels very fresh and fun. Maybe it's because it's based on Sega games I don't have much memory of, but I find many of them very interesting. I especially enjoyed the level from Sega's After Burner game, where you find yourself on the decks of aircraft carriers in a naval fleet, and naturally there are jets racing by as you boost and take off from the deck in your own jet, or dive into the ocean in your speedboat. I haven't unlocked all the levels yet, but there were many others that were just as unique and made great use of the plane / boat mechanics in addition to the karts. As in most games, there's a stinker or two in the mix (I didn't really understand or enjoy that level that looks like the inside of a Mexican restaurant), but overall most of the levels were really great and unique.
Another nice feature of this game is the way items work. In Mario Kart, the items are not random at all. If you are in the back of the pack, you get huge items that can take you right to the front. If you are in the front, you get the pathetic single bananas. This keeps everyone together, which does keep things competitive, but at the same time can be really frustrating. Have you ever run a great race only to get shot with three blue shells right at the finish line and sent straight to last? That really doesn't happen in this game. You can get good items anywhere in the field, and the items aren't as overpowered. They typically knock you back only a spot or two, so the game is much more skill based than item luck based. Sometimes that's not always a good thing, because there can be a lot of hilarity involved with huge Chain Chomp or Bobomb explosions in Mario Kart, and this game will never feel that crazy. But I think there is a place for a skill based Kart racer, and that's one area where this game distinguishes itself.
One other thing I'll review before closing out is the multiplayer system. You can have up to five local players playing, which is awesome. And there are many modes to pick from, from racing, arena battle, battle race (like racing, but also like battle in that you have to survive to the finish line), Capture the Chao (kind of like capture the flag), etc. One thing Sega got right, or at least got closer to getting right than ANYONE else, is capturing the feeling of Mario Kart 64 battles. Nintendo has done an absolutely lousy job with battle mode since Mario Kart 64, as anyone who remembers that game can attest. In Mario Kart 64, there were several 3D / multiple height levels, where you could get above your opponents and fire down on them below. That added a great level of tactical depth and extra fun. But unfortunately since then Nintendo just makes flat levels, as though we all want to play bland SNES levels still. This game has a couple battle levels that are multi-height though, which adds back that 3D tactical gameplay. It's not quite as good as Mario Kart 64 for a couple of reasons though. First, the items aren't quite as exciting (again, this is good in racing since it makes it more skill based and less about item luck, but not quite as good in battle where you are looking for craziness and explosions). Second, not all of the items make good use of the multi-height levels, as some self destruct when trying to fire them over an edge to hit someone down below. However, not all of them self destruct, and for some that do, you can often leap off the higher level and fire the moment you are in the air, and then they will be successful at launching from a higher height. But again, I have to give Sega props for FINALLY capturing part of what made Mario Kart 64 battles great, since apparently Nintendo is a lost cause.
The last thing I will mention about this game is online multiplayer. I've played it several times, and there seems to be a good ranking system in place that will match you with a suitable opponent. And the game played well and I saw no lagging or framerate issues. However, there is one problem with multiplayer... very few people seem to be playing it online. I've never been able to get more than one opponent into a game with me, and sometimes it takes quite a while for the system to even find one person available to play. Contrast that to Mario Kart on the Wii, where even years after release I still have no trouble finding lots of opponents and playing with very full racing fields. So just be forewarned that you shouldn't buy this game expecting to have tons of people online to race against.
But since single player mode is deep and long, and five player local multiplayer looks strong, I can still heartily recommend buying this game, especially since it is only $22 right now. And even after Mario Kart comes out, I think this is still going to be an excellent, different game that can stand on its own right beside Nintendo's entry.