It's mildly entertaining. My critisism both ways in a list:
Good:
Engine is mostly stable.
Animation is clean and smooth.
Quite functional as a basic platformer engine.
Bad:
Not much "Sonic" here. One could easily replace the sprites and get a Mario game.
"Fangame Collision Syndrome" a nasty, yet all too common tendancy of fangames to have poor collision routines, that makes the player stand on walls.
There's no decelleration, which can be tricky at times in the platforming.
Topspeed is a bit slow for a Sonic game.
Holding jump allows you to rapid jump (I think just changing the action used will fix this in GameMaker)
Don't let the longer list of Bad fool you, this is a midly entertaining game, and quite acceptable as a random demo.