Not replying to anybody, just thinking aloud:
I found Unleashed to be a far better game once completed and all stages were unlocked, or at the very least everything was completed up until Eggmanland. I've just reached Eggmanland, and found that the game is far better now that I am able to jump around between day stages, playing the ones I enjoy. Also, I can finally play all the the DLC stages which are, as Sharky said, better than some of the on-disc stages. Even though I'm only replaying 30% of the total game plus DLC, at least I'm enjoying a solid chunk of stages that are equitable to the amount in SA2.
It's just a shame that one has to play through most of the game, putting up with the 30+ minute werehog stages and the tedious medal collecting, in order to reach that ideal spot where you can basically level select the good stuff and play the DLC.
Some could also argue that Sonic Advenure 1&2 are better games once you have all the Sonic stages available, as you can just play through the trial menu of SA1 or jump around the map of SA2 playing Sonic/Shadow. However, I think that unlike Unleashed, the non-Sonic segments of SA1 and 2 are so short that even if you're hating on having to play as Big, Amy or E-102, their stories could probably be beaten in the same amount of time that it takes to complete three of the longer werehog stages. Same goes for SA2, I don't recall the treasure hunting stages or mech shooters taking all that long before the next Sonic/Shadow stage hit.
Long point I think I'm getting at: Sonic Colors seems to be, based on how much I've played so far, the FIRST 3D Sonic game where what I'm playing in the main game is all of the same content that I could see myself returning to once completing the game. No stage or act looks to be non-Sonic filler that I play through just to get to play the upcoming Sonic stage.