Hello;
I played the game however I had some serious FPS trouble. I'm not sure if it's a Gamemaker thing as I sometimes have trouble with it. But the game runs very slowly for me at times. It gets a little better with time but that's also me getting a little more used to it.
Also, it looks like you've got enough stuff in place to finish it. All you need to do is throw in another level or two and think up some boss fights. I'd very much like to see it finished.
Giving up early on I could understand, but it looks to me like you've got most of the game thought out, and the means to imagine it in your game. I know there's always the "developers block" but I see no reason why it would be hard to overcome in this case.
I think it would have been cooler if you gave the Sonic you play as armour though, maybe differently coloured from the alternate Sonic. The Advance sprites are overused but I liked the Sonic/Emerl fusion a lot.
Also, Sonic needs to be a lot faster. I know this isn't a traditional Sonic game, but there's no point having Sonic in it unless he at least has speed(and maybe some kind of spin move too?).