This game might have been a port of the Sega Genesis version but it holds its own on the SMS. The difficulty is just right, there are bonus stages and even emeralds to collect for some replay value. The graphics are also some of the fastest moving and most vibrant of any SMS game.
I would say part of the reason Sonic became so popular wasnt only the Sega Genesis, it was the millions of SMS owners that used Sonic as an example of why the SMS was better than the NES.
I had to save up for months to buy this game but it was worth it.