Single player is a blast, is multi any good?

Yes it is. Played it for hours with a friend of mine and we had a lot of fun.
You got to split the loot though, otherwise one will become op while the other needs to run for his life constantly.
Playing extra levels before the last stage can be pretty messy. The game will just continuously spawn bosses at you (till the countdown of the portal ends) and you'll either die by mobs or lag in most cases (we once had ~10 magma worms at a time + others).

Also note worthy: some characters can only be unlocked in singleplayer.

Thanks, can't wait to try it!