Huh. It's actually not bad. Couple tweaks here and there (soundwise, anyway), and you have yourself one hellova project.
I like this game. I really like this game. In fact, I think it's my new favorite flash-based game (not that I ever had one to begin with).
I enjoy how it borrows elements from all of the different games. The way they did the signs is also neat (by having the actual content on the sign instead of a generic graphic for all of them). I also enjoy how it utilizes wide screen monitors if you are using one.
I do have a few complaints though, most notably the poor frame rate that I'm getting when I'm maximized (which is understandable, since having flash render something so complex in 1280x1024 and 1280x800 using only processing power and not using GFX power is a bit inane) and how Luigi (if you select him) occasionally switches to Mario for a few sprites. The control setup feels a bit odd as well, but after a while of playing it becomes a non-issue.
Overall, it's excellent. It's already amazingly great in it's current form, so it'll more than likely be insane when it's 100% finished. I give it two thumbs up and a box of cookies.
Pretty kickass. It's wonky for the up arrow key to be mapped as jump though.