Did anyone manage to complete the game? I might be missing some endings because I got stuck, should try again :\
Yes, twice, but I haven't finished the Hard Mode or gotten all 300-odd routes.
The game is ridiculously gritty and has many poorly applied ideas, but I still find it a blast to play and way more engaging gameplay-wise than Heroes. One of the main problems is that some missions suffer from difficulty spikes where it's hard to know what to do or where to go, but if you DO then they aren't as difficult. Central City and Lost Impact's Hero missions are amongst the worst offenders, though I can do both now fairly easily.
I think that your enjoyment tends to depend on how much you care about Shadow's story or how much the gritty style bothers you. I just ignore the latter and enjoy playing it.
Also, unlike Heroes before it and many other Sonic games, it actually has somewhat competent grinding controls, at least they work fine for me. Better than in Heroes where switching rails would always be a risk of death.
It's nowhere near as broken as 06 is though (from what I've seen).
EDIT: Oh, and on the Gamecube, at least, but I'd imagine that something similar exists on the other consoles, you can change missions by using the directional pad. Makes things much quicker.
I have a question: Am I the only one who doesn't want either the "blast through levels really, really fast" that Sonic seems to have become, or the "too slow alternate gameplay styles everyone hates", but rather more of a balance between the two i.e. speed-based gameplay, but maybe with OPTIONAL alternative characters (Tails, Knuckles, etc) to give you different ways to play. Essentially Sonic 3 & Knuckles in 3D?
I'm probably alone in this, but I prefer Sonic to be more a speed-based platformer, not a racing game.