If I may play devil's advocate, I will counteract this and describe what Sonic 2006 did right or what they aspired to do right but were plagued by deadlines and not swallowing their pride to release it way later and ensure it was completed properly.
Gameplay? Plagued with glitches and bugs, yes. Running against a wall and suddenly falling off of it. Riding on boards impossibly terrible. Mach Speed Zones, bad. Diversity in gameplay? ...actually rather impressive. One thing people gotta keep in mind is that this was a spiritual successor to the Sonic Adventure series. And in those games, Sonic wasn't really that fast either. Only time we really got to see his speed was in the likes of Speed Highway or when we abused the spin dash, which they practically took out in this new game which I did not like. The power-ups were cool and the homing attack system was done alright. How to fix Sonic's gameplay? Get rid of glitches that cause unnecessary deaths, fix riding boards and the like, and turn Mach Speed Zones into the style of Sonic Unleashed/Colors/Generations with speed boosts and all. One problem down. Coulda been executed a lot better for sure.
Shadow's gameplay and Silver's gameplay were the best realized, in my opinion. Shadow's combat system with handling multiple enemies was satisfying and I loved warping between a hoard of enemies and homing attack into melee combos. If it weren't for the long loading times and the occassional glitchy stage sending me to my death, I'd fully say that this was the best we've ever seen of Shadow in terms of gameplay. No guns (which weren't bad persay in his own game), just Shadow racing to the end while utilizing his chaos powers to take down his enemies. ....and I miss Jason Griffith's Shadow. Not gonna lie. (disclaimer, I was a supporter of most of the 4Kids cast. I don't frequent Sonic boards for a reason. those fans are bat**** insane)
Silver? Another realized character. He could serve to be a slight bit faster, but otherwise, it was awesome. I agree with Grimm as far as that goes. Most of the additional characters needed more time to be developed though as far as gameplay went. The best one was Blaze, who I felt handled even better then Sonic did.
A little sad, but either way. I would love for them to revisit this game and remake it to be the game it was truly meant to be. That or when they release Sonic Adventure 3, use 2006 as a foundation and build from there. Myself, as long as I get Sonic and Shadow I'll be content.
In defense of the replay value? It's hard to keep replaying a game that's so damn slow in loading times. It kills your gaming buzz, and you need some serious patience in order to beast your way through it all. Otherwise I'd play it again and again if it had none of the problems mentioned by many of the fans. Music? Freaking awesome nothing else has to be said. Good job as usual, SEGA.
And you guys are going to call me crazy for this last one, but the storyline? ... I liked it. I liked the conflict between Shadow and Mephiles ("If the world is to become my enemy, then I'll fight as I always have."), I like Silver being naive and learning the truth about what killing Sonic would do. I like Sonic and Elise's story.
... Yup you heard me. You're all creeped out by the whole thing? The internet's ruined the minds of so many people, so hear me out on this one. It gets borderline uber lovey-dovey and corny, I'll admit. But let's present a couple of examples here. The Princess and the Frog. Beauty and the Beast. (yes they're humans to begin with, but the point comes here). You're engaged with the storyline of them coming together despite their differences, and they love them for who they are (even if they're rewarded with a handsome prince afterwards). And no harm done in that Sonic was able to protect Elise, and when Solaris was defeated and they erased their meeting, it was a vague familiarity as Elise returned to her normal life. Sonic is back on a new adventure, like the wind, there one second and gone the next.
Here's the thing about the Sonic & Elise thing. Don't take it so seriously and try to look at it from the more innocent Disney perspective. It's not like it canonly happened anyway (well technically it did but time erased the events for the sake of the future).
...and for all the things I've defended Sonic 2006 for? I am never playing it ever again.
The loading times and cheap deaths via glitches just KILL it for me. Eff this game. I love me some Sonic Generations.