I genuinely disliked Sonic 4's physics a lot, and Classic Sonic's physics in Generations, if I were being nitpicky, still have a few problems (as spindashing pointed out). As Anth0ny just pointed out, the only way SEGA is going to be encouraged to even develop Classic Sonic's style of play even better is to purchase a game like this. Do note that today's younger Sonic fans will buy anything, especially if it's related to Shadow (a very popular character among that age group from my experience) or the Chaotix. The Classic fans seem to have a smaller voice.
I still appreciate what Sonic Team did with Colours, Unleashed's day levels and Generations' level designs and presentations. I'm still having fun with it and that's what matters to me in the end--letting go, having fun and enjoying the crazy fanservice. In addition, if there's one thing I can't knock against Sonic, it's that it consistently has a wonderful soundscape (Chronicles excluded).