Pros:

Amazing attention to detail.  The levels are pretty much exact recreations.

Actual music and gameplay mechanics, like platforming, the bouncing crates, the timed TNT crates, and the sound effects to go with them. 

Great feeling of nostalgia was almost overwhelming during the first few levels.

Cons:

The level 3 bug.  Can't give it a 10 because of that.

Conclusion:

Amazing experience.  Mods like this are the very reason I game on PC more than consoles these days.