Great little game.
I liked this game because I am a fan of sprites and Sonic games. The graphics are pretty self-explanatory, although I found distasteful the combination of 16-bit and 32-bit sprites on the same screen. (I understand that it may have been necessary, but nevertheless.) The background song got kinda annoying after a while -- put in some way to turn it on and off. The only violence in the game would be that of launching the characters, as well as them hitting the floating mines and spinning blades. I found the poses the characters strike when they hit something rather humourous.
The game seemed to get boring after I got all the characters and the emeralds. And by the way, I'm not sure what you mean by "secret ending." If you mean the one with Eggman (I don't want to give it away), I finished it.