I see it from time to time in those bargin bins at used video game stores and pawn shops.. Although i must disagree with the above post.. cause its a great game!
It was built into some of the later (or was it earlier?) model 2 Master Systems instead of Alex Kid.

I have a copy of the cart, although I got it when the game was new, but I do see it periodically on eBay Australia. It can't be all that rare.
i seem to remember that alex kidd was around before sonic 1, so id guess it was the lattrer sms'es that had sonic (im pretty sure he was the replacement for the rather unsuccessful alex kidd mascot)
I was quite fond of Alex was all I played until I figuered out how to play Ghostbusters
It's a damn hard game to figure out...I used to be able to deal with Marshmallow Man when he turns up. If anyone could remind me how to stop him, I'd be greatful...
I have Master System II with Alex Kid (in conosol) and Sonic. The game (sonic) is very good for MS, and the graphic is awesome, for 8 bit console. Variety is not like the Sonic games on Genesis/Mega Drive but is not so bad especialy for the 8 bit system. Pity is that my MS is sunken in dust ???
If you mean Sonic, then I think there are a few minor differences. For example, because of the screen resolution of the GG it is possible to jump up and hit Robotnik as he is flying overhead (first boss). In general, the game is more or less exactly the same.
It is possible to hit robotnik on the sms version when he is flying over the top on the first level. You just have to time it right. I didn't thinkit was possible untill i did it the other day
Yes it is possible, but I think you need to hit him a few times first. It is very easy in the GG version.
I think it was not very rare here in Europe, I had the cartridge while some friends of mine had a SMS II with Sonic built-in. IMO it's one of the best Sonic games EVER. Maybe Sonic 2 was the best in SMS, but Sonic 1 is my favourite.

Mmm there are people who don't like Sonic 1 at all... what did you think when you saw Sonic Chaos Masterpiece ?
Choas was a nice game, had some pretty nice effects/graphics. The bonus stages were mostly pretty fun (except that stupid spinning through tunnels one). It suffered from the long standing "another Sonic game that is way too easy" syndrome, that plagued almost the entire series though. A really good showcase of the SMS capabilities.
Yeah, its graphics were good (although they suffered a bit of slowdown in the PAL version), but it was just too easy and you could complete A LOT of stages in a few seconds. In fact, it was one of my Christmas presents and I found it inside my mother's closet two days before :
: and completed it that afternoon, hehe.

Bonus stages were also decent, they were the only difficult task to complete in the game and their variety was interesting. However, SMS/GG Sonic 1 had the best bonus stages, and they were much better than that rotating stage found in Genesis Sonic 1.
I loved those Sonic 1 bonus stages

It's probably where the idea for Sonic Spinball (or Sonic's Pinball) came from.