going through it again on the Solider after finishing it with the Siren - have to admit im liking the solider alot more! hit lvl 30 last night before i had to forfeit the TV for cr@pfactor.
the guy who i did my first play through with (he's using the Hunter) decided to start a second play through with him and said the difficulty increases quite a bit which is good, he hit lvl 40 before leaving firestone again.
what i will say is the drastic drop in item quality from co-op to solo is really noticeable!!