Funny this should be asked, I just did one this past weekend. Normally I don't do "killer encounters" but in this case the party made a Wrong Turn (tm) and was headed towards the deeper part of the dungeon before they were ready. The first encounter they had was noticeably tougher than the goblins and hobgoblins they had been fighting but thanks to some lucky rolls by the barbarian it wasn't quite as much of a challenge as it should have been so they didn't pick up on it. Then they encountered a party that outnumbered them almost 2-1 with obvious spellcasters and healers. And they attacked and fought anyway, and only quick thinking by the gnome threatening to blow up the cultists to hell and back with a smokepowder keg saved them from a TPK.
Generally the only use for such encounters for me is as 2 and 5, which is what the previous encounter was for the most part.