I finished Sonic Generations a week ago and I'm still playing it. I'm six Red Star Rings away from unlocking the whole collection room. I just have to get two as Classic Sonic and four as Modern Sonic, all in Planet Wisp.
Also, Planet Wisp's two acts are the only two S ranks I still need on the main stages. I don't think I'll be doing the boss battles or rival battles for S ranks. I had enough trouble with Silver and the Egg Dragoon the first time.