Note that there're mild spoilers in here, such as the numbers of tracks and length of story mode and such. If that bothers you, you probably shouldn't have clicked on a topic named 'Riders' to begin with, so stop reading. D;
Also note that my opinions on the game are not at all guaranteed to be the consensus. Others who have played it should post what you think too.
Anyhow, I think it is great. I've played it 18 hours since I got it yesterday. (A large part of that is also that I have been sick :P) I've completed about 75% of the game (excluding perfect rankings) and read about the rest, so I think my opinion is pretty well settled. I find it to be, oddly enough, the most Sonicy game I've played in a long time besides Rush. It also borrows from Rush, as the main thrust of the gameplay, the air meter, functions highly similarly to it, depleting with boosts (and most other functions) and recharging with tricks.
I find the game endearing because the controls are relatively simple and yet the game is wicked hard. The simplicity doesn't mean it's straightforward; the game gives help as to what buttons to press to get around curves, but even so you can expect to spend a fair bit of time eating the wall in 8th place until you've gained experience with the physics. The game reminds me of F-Zero GX in several ways, and one of the foremost is the steep learning curve. The story mode is short enough to conceivably be finished in half an hour, but the first time through I averaged that long on each course. Oddly enough, for whatever reason, the only course I managed to beat on the first try was the last one.
The other main mode is the missions, which (as far as I know) gives five objectives for each course. They range from stupidly easy to ridiculous, especially if you try for gold ratings, which you'll have to if you want to unlock some of the coolest stuff in the game. And they're awesome in that unlike the obscenely long Shadow/Heroes missions, few stray much over a minute long. This makes a world of difference as far as I'm concerned. The courses themselves are limited in number - 16 total, in two pairs of 8 that match up somewhat analogously to Acts 1/2 in S3&K - but well designed, and the last unlockable one is simply awesome.
There are a few negative points to Riders. There are some unexcusable omissions from the character list (I'm looking at you, Metal Sonic). The presentation could have used another layer of polish, as there're several pointlessly annoying oversights, such as TA course selection only showing your fastest lap - you have to leave play mode and go into the separate records area to see your fastest total race. And each course has a segment which refills your air as you madly rotate the control stick, which will kill your hand if you're not careful.
Overall, though, it's the best game Sonic Team's put out in years. I'll be highly excited if/when Riders 2 is officially announced.
