Three days ago, my friend, brother and I bought and played through this game. I was inspired to do a playthrough by the SA crew that LP'd the game, but decided to do it over the course of two days in multiple sittings. My brother left a level into Silver's story, which we started right after completing Sonic's, so my friend and I were left with the bulk of the game.
I was honestly surprised how much fun I had with this game. It definitely has it's bad points, yes. The mach speed sections are tough to control, but even so I had little trouble with them (I was given every mach speed section but the first, and only died twice somehow.) This is of course non-indicative of the average player's experience, but it's proof that it can be done, especially if you know what you're getting into. The story is a convoluted mess with uninteresting characters and poor voice acting, but Sonic games before the Adventure series rarely relied on story. The physics engine is oddly implemented and leads to unfair deaths. Some levels last entirely too long for a Sonic game, and the same can be said for some bosses. The camera will attempt to murder you at every corner.
However, the game has some genuinely redeeming factors. The soundtrack is great (even though you might not be able to hear it sometimes.) If you get used to the engine, the game offers some fun sections. The best reason to play this game is the unintentional hilarity, though. Between narmy lines ("That alone is insufficient!", Shadow's scream of "PERFECT!" upon S-ranking something, etc,) unexplained plot problems (Why is the Egg Carrier randomly crashing? Where'd that Chaos Emerald go?) and some of those cruel deaths mentioned earlier, the game is great for those who love the So Bad Its Good
Overall, it's worth a playthrough, especially for the price. It's extremely cheap on Ebay and only around 15$ elsewhere. Grab a few friends to play it with. If you go into the experience keeping in mind it's far, far, FAR from perfect, it can be pretty fun.