People say SMF sucks because they can't code, in my opinion.
Put it this way, I could code a whole site-forum bridge and a load of SMF plugins in a day or two.
Try do the same thing on something like phpBB or vBulletin and it'll take longer (because you have to read up on all their hooks and such).
Due to it being nicely coded though, it means it doesn't look as good as things like vB (probably because it doesn't waste queries on fetching useless information purely for cosmetic purposes).
It does mean it is rather fast, though.