I think the main problem with the level editor that people seem to be having is that it's just a parameter for the game's EXE, not a seperate executable entirely.
Anyways, the only massive problem I have with the current editor is that waypoints are an utter bitch to work with, and the game crashes if you touch Bandage Girl. Otherwise, it's prettu decent.
Just tried to save a good level on devmode... it failed!!!! Devmode sucks!
Make a directory in your SMB folder (C:\Program Files\Steam\steamapps\common\Super Meat Boy) called "levels", it should save then. This is also another problem, why can't the game create the directory itself when you save?