Pretty damn good.
I've been playing this game everyday for about 3-4 days now and I'm tempted to do a let's play of it. if I get the chance, I will. You did a good job making the game fun.
For Phantos, yes he is a bitch to dodge, yes he makes boss fights a lot harder, but if I can beat Wendy Koopa while dodging it (and mind you, my reflexes suck) then it can't be THAT hard. It's really just a matter of passing it before it has a chance to lock on. On later levels though, when you don't get the check point before the key, it really just becomes too difficult to accomplish for the average player unless they abuse the boo power up or the pause trick. I would really suggest working on how you curve the difficulty, because it seems in that while most the game the levels are relatively simple and can be accomplished after a few trials and errors, there are suddenly insta-kill levels in which you either have to have cat like reflex or utilize the cheaper abilities.
Level editor works really well though if there were more obstacles and enemies available to choose form that'd be great too. Just something to think about. Also there should probably be some kind of sharing system aside from copy paste your code and post it here, maybe like a forum or something. If it exists already, never mind. I just thought I'd suggest it since all that's happening here is they're getting voted useless and being pushed further into the review pages.
Music was great, graphics gave it that nostalgic feeling honestly, It's a good game and I'm probably going to be playing it for a few more days to come.