JEV3 created a Megamix thread back in August of '08 after the recent events that the people who created the game decided to end the project short. The thread wasn't locked; just another case of thread inactivity, after about only a month after it was started.
I haven't really played much Sonic Megamix. It is an ambitious attempt to rework Sonic 1 (Genesis/MD), and it's a well-known and well-praised Sonic hack. I'm sure those new to modifications would play this and like it quite well. I haven't gotten the Sega CD version to work, but I liked what I played so far. It doesn't really matter that much if it's far from an ideal release/incomplete, just as long as it can be fun to play (Q. E. D. -> SRB2).