I just purchased Sonic 4 this evening and am loving every moment of it.
I noticed a lot of my friends from here on XBOX live have already completed the game. I decided to make a promise to myself that I would only play one zone a day to stretch that 15 dollars.
I absolutely love redoing the levels and trying my best to beat my old times and scores. I even have decent rank-ins at the moment. I will try to stay at the top 100 for some levels just to make myself feel special. Kind of like what I did when I played Outrun Online Arcade.
Man, the end of this year is really shaping up to be a good SEGA year for me. I got Bayonetta, Afterburner Climax, Sonic 4, and soon Crazy Taxi HD. Aww man, it finally feels good to spend money on download only games.
edit: I can't believe I forgot Sonic and Sega All Stars Racing. XD