I've got 62 hour played on Steam. I've beat every boss and played through a couple of times solo, the rest coop with randoms.
IMHO, without any doubt whatsoever, this game is far easier solo than with other players. Sure, there are a few boss fights that devolve into a constant rollfest, that I've thought they could dial back a bit for solo players...where you barely get a moment to dps the boss in between wave after wave of adds (ie Mangler). The trade off for going at it with other players is that now it takes that much more to burn down the boss and if the other player(s) don't know how to play, well you're only making it harder for yourself.
The end boss- I beat it solo with little effort whereas I've been in only one successful group out of dozens. I just leave at this point where I come into a game where the host is at that point. Waste of time.
GT/PSN: Evel138 My gaming Space http://imgur.com/a/GjHxd