I don't really like most of the ports, when I play them I just want to play the Genesis version instead. That said, the Sonic games aren't ports, they're new games, and they are mostly great. I particularly like the first game, it has some great tunes. Also, the SMS versions of these games are better than the GG versions due to increased resolution. That first boss of Sonic 2 is near impossible on the Game Gear and a piece of cake on SMS.
I always heard Castle of Illusion SMS was better than Genesis, but I've never played that one.