I completed this game on the day of purchase on Wednesday I believe no side missions except one to get the Boss Keys and Chaos Emeralds, I thought it was kinda short even though I played through both Classic and Modern Sonic's stages completely.
It's definitely one of those "essential for the fan" games. While it is short, the game play is fun overall (though the frame rate can be a bit tedious)
I thought it was one of the better Sonic games made in the last few years (Never beat Unleashed and never played Colors), with the Classic stages for nostalgia and Modern for those speed-runners. Hopefully we'll get some DLC soon.
I know this isn't exactly the place to ask, but has anyone else encountered the issue with the Steam version not working on Intel laptops? I'm trying to figure if there's anything I can do about this, or if I've wasted my money (Not that I wasted much, lol Christmas sales). All I can say is, it looks exciting. And Hard Mode actually looks slightly challenging, which is nice. Story seems kind of a throw-away, though.