It wasn't completely terrible, but it was the beginning of the slippery slope that led to Shadow the Hedgehog, and Sega's attempt to atone for it with Sonic 2006.

Sonic Colors and Sonic Generations are great. But then again, I'm so used to mediocrity on the Sonic franchise

Any Sonic game post Genesis is a waste of time.

Cherry Pop Culture: Sega has been slowly clawing their way back up from the dual nadir that is Shadow and 2006. They still haven't reached the level of the original games and may never do so. Just like the now-ended Land Before Time movie series.

I just hope, that deep down, the newest games really are good. I spent too much time being drowned in crap not worth playing, anything not garbage already looks amazing to me.

Knuckles' Chaotix for the 32x was really the beginning of the end.

Sonic adventure was a good game, but I feel perhaps that at that point, the entire series fell into the yawning rift created by Super Mario 64. Following how Nintendo managed to take a 2d game and turn it into 3d. The problem is that Super Mario 64, for all it's innovation, was not really Mario. It's only just within the last few years that game designers have started understanding that two dimensions are not a limitation.

Nintendo has figured this out. There are now as many side-scrolling games with Mario as there are true 3d ones. Sonic is still struggling with his lesson.

They made a shitload of 2D Sonic games in the latter years, some of them very recent. Most for handhelds, but not all. They are usually better than the 3D games, but that's not saying very much.
Even back in the day there were an assload of crappy spinoffs - people always seem to forget the isometric games, the Game Gear/Master System games etc. Sega ran Sonic into the ground in record time. Really there are only a couple of truly great Sonic games, and one of them is two games piled on top of each other. I can't even count how many Sonic platformers they churned out in the early 90s. It must have been, like, 2 or 3 a year? The difference, of course, is that some of them were good - now the few remaining people who buy them have to make do with 'kind of decent, considering'.