I got the crazy idea to start playing Sonic Heroes last night. I played Team Sonic all the way up to Mystic Mansion. I'll probably write a more thorough summary of my thoughts later, but generally speaking, I enjoy most of my time with this game, and find myself revisiting it with more enthusiasm than the Adventure games, BUT that's only cause I have gone back to those a lot more recently. 
I like the level themes, the return to a simpler story, and the team mechanic. It's a lot more light hearted in style, and presentation, and honestly, at times feels like a truer successor to the Genesis games, from a thematic standpoint. There's also no bullshit fishing, or shooting. It's mostly levels focusing on platforming, zooming through set pieces, and traversing some of the most fickle grind rails in gaming. I find the game really fun, when it wants to cooperate that is. One of the biggest, glaring problems I have with this game is way momentum is carried into a wall, forcing you to run parallel to it causing numerous deaths if you're not paying attention. This can be compensated for, by jumping, but still, this wasn't a problem in the Adventure games so why is it here? Also, fighting with the Power characters advances you a few steps up, and it can be easy to send yourself careening off of an edge when fighting a lot of enemies on an open platform, something you'll be doing a lot late-game.
The other major issue I have is with some of the level designs. In particular, uh, well, basically the entire back half of the game. There's seven zones, with two acts each, and the fourth zone, Rail Canyon/ Bullet Station introduces us to rail grinding again. Okay, technically it shows up for a second in Grand Metropolis, but it's not the focus of the level there. This is fine in and of itself, it's a fun way to move around, but the problem here is there are entire stages where you are doing this over bottomless pits, and the game can seriously just fuck you right up sometimes. It's like they forgot about regular levels. Frog Forest and Lost Jungle also have a ton of grinding, and so do Hang Castle, and the Egg Fleet levels. I think this would be fun if it worked correctly. A game that doesn't work 10% of the time is a broken game. (See my rant about motion controls in SS). You also have to have split second reactions, which is fine and a staple of the series, but when you get dicked by the controls, or miss a rail, or get thrown off somehow, it is infuriating.
You're not dying because you aren't good, you're dying because of shoddy coding, and that is what it so bad about it, and why people harp on this game so much, imo. This is compounded by loooooong stages, and a small number of lives. I got to Rail Canyon with 11 lives, and game overed twice (you start with 3 lives, but it's really 4 cause 0 counts). I'm not saying I'm a master of this game by any means, and surely I'm not blaming all my deaths on glitches. The game works most of the time, and a good chunk of deaths were my own fault, but the ones that aren't really stick out. This is most apparent in, here we go, Rail Canyon, Bullet Station, Frog Forest, Lost Jungle, Hang Castle, chunks of Mystic Mansion, Egg Fleet, and Final Fortress. Oh, is that over half of the game? Hey, the first 3 zones are really fun. Even Rail Canyon has some "wow" moments, but yeah.
Then there is Mystic Mansion. Oh my God, this is literally, bar none, my most hated level in any Sonic game. It goes on, and on, and on, or focuses on combat, and puzzles which are clunky at best here, and the combat is a boring mash fest. Then there's that part at the end, which has some precision platforming, and after you are 19 minutes into a level with 0 lives, that Game Over can be a real controller snapper. This stage also brings out another issue, where there are so many jumps that are literally do, or die. This game is hard, and loves to throw you into pits for missing one jump, even when the controls are working properly. Hey remember when missing a jump, or messing up a speedy part just dropped you into a lower part of the stage, usually not insta-killing you? What happened to that?
By now, it probably sounds like I hate this game. But, you must understand, I am a Sonic fan after all. I could complain about the padding in the form of requiring you to play the same levels with little difference four times to get the true ending, or I could whine about the utter lack of good bosses, or I could whine about a lot of stuff, but all in all, I do like this game. Understand that it has some serious flaws, and can, and will troll you, but it's not a horrible game. It could have used a little more time in the oven to iron out the kinks (a sad reality for the string of Sonic games to follow). As an aside, I would have liked to see Generations use the Casino levels from this instead of Sea Side Hill since you'd have a pinball level, and not another "first level" tropey area.
I'm going to be getting Freedom Planet soon too. It's been on my radar for a while now, and it looks really awesome.
Oh, and don't ask me about the special stages. I'm not on speaking terms with them.
EDIT: Started a Team Rose playthrough to grab the Chaos emeralds. I actually like playing as Team Rose, for a few reasons. Mostly, Big, in spite of being derpy as all hell (and voiced by the guy who voiced Duke Nukem) has some good attacks that pretty much destroy anything in your path. Team Rose's stages are a lot shorter, but they feel more in line with the length of the older games. No area really over stays its welcome, and it's a less tense, more leisurely experience. But seriously, why do Cream and Amy have to say "Here we go!" every 10 seconds? You're not Mario. I do find the banter to be a nice touch for the different teams, but sometimes, it's a bit much. Also.. why are there so many.. panty shots of Amy? Isn't she like 12 in this game? It's so much easier too. It is pretty much the game's easy setting. I have 30 lives at Rail Canyon/ Bullet Station's boss, to put it into perspective.