Loved it.
It was simple, fun, challenging. And oh my god, those extra levels we're really hard. I love this kind of games, at the first second I started playing this I was like "Super Meat BOOOY!!!". I'm not such a fan of story or something, I'll always go for the gameplay. But I have to say, you've got some really good thing going on here, add some stupid or fun story, and you're done. Maybe some level creation for a further release or something. I have to admit it felt really short. Got trough the campaing and the expansion levels on less than 20-25 minutes. And please, people, stop being such whiny babys about it being too hard. "Oh noes this makes my computer lag and the jump lags on me" PFFFT, BULLSHIT.
In the end, awesome game, you could add some more music and make it longer, and it would be perfect. If you would add a "Level Creation" it would be top notch. ;D