Sonic on the Master System is a great game. It's not as good as the first Mega Drive game, and Sonic's movement is a little floaty and imprecise, but the levels are designed well around the character and movement. There are some big changes to gameplay, such as the ability to collect rings after you've been hit, and the collision detection seems a little off as well.
The levels are similar, but not identical to the Mega Drive game. The fist two levels - Green Hill and Bridge - are a bit too samey, but after that the scenery changes, sometimes aping the Mega Drive's levels (with very close similarity to the Labyrinth Zone and the Scrap Brain Zone), and sometimes with a completely separate feel (the Jungle Zone has never been revisited, as far as I'm aware).
The special stages on the Mega Drive were where the hardware was used to maximum effect, and it's here that the Master System is a bit of a let down. Built around different coloured springs, but with the game moving a little too fast to be able to judge where you're landing, this felt too much like a random mess - more so than the Spring Yard Zone or Casino Night Zone, and even more so than Sonic Spinball.
Some of the bosses seemed overly simplistic, while some were a little more tricky. The Jungle Zone boss was one of the more tricky ones, but not because of his movements, more the slightly clunky jump that Sonic had from the angled platforms at each end.
Some of the later stages worked really well, particularly those that forced Sonic to slow down - maybe belying the fact that the Master System wasn't built for Sonic's usual speed. With a slower tempo, the level design was massively changed with many more rates ad hidden parts, and the jumps made more difficult. Lightning flashed around the level making Sonic wait for it to clear. This was, perhaps, my favourite level - the Sky Base Zone had some similarity at times to the Wing Fortress Zone at times.
And it all ended with a pretty disappointing boss fight. Standing on the far left of the screen, as below, nothing could hit Sonic, meaning he could just wait for the electric barrier to drop and dash over to hit the glass tube. A few hits later, and he was down.
No, I didn't collect all the chaos emeralds. Yes, I probably will one day. Yes, I'm counting this as completed.