I think the older Sonic series was better. I can't say I am satisfied with Heroes or even the Adventure series, I still think Sonic 3 and Knuckles was the best Sonic game, yet. It just worked out better back when it was a 2D side scroller, and you didn't have to worry about a bunch of bugs and some poor gameplay.
I liked the Adventure series because it was mostly story driven, and I happen to like that. Heroes was more of a disappointment. It had glitchs, bugs, and could've been a lot better if they spent a little more time on it. I didn't like it's Special stages, either.
Another aspect that most people don't seem to notice, is the music. I really don't care for any of the Adventure series or Heroes music tracks. I loved the music from just about every Sonic game for the Genesis. (except 3D blast -_-) The music from Ice Cap Zone, Lava Reef Zone and The Doomsday Zone from Sonic 3 and Knuckles are probably my favorite.