Excellent!
This emulates the original almost perfectly! Wonderful, wondeful, wondeful is all I can say. I do have 2 very, very small gripes, however:
1. Pipes. They made Mario Mario, and I want some to go down.
2. Those blue shelled underground guys. If I recall, fireball's won't actually hurt them in the original game. I might be wrong though.
Other than that, (to quote the Smipson's Comic Book Guy) Best. Game. Ever.