Nope.
I found it to be a blast single player, and the co-op is just an added bonus to my experience. Better loot drops, a bit of banter and somewhat social interaction, what's not to like.
You just need to get a firm grasp on the mechanics, especially the skill trees (every point counts) and the weapons you use at any one time during your game.
I'm happy with my lvl50 Siren, which I still have to gear up and re-spec depending on what I/we intend to go and do and what my dedicated task will be when playing with other players. Proper communication makes things so much more easy.
I won't buy any more DLC until the level cap goes up. If it doesn't, hey, 'twas a fun ride. I tried the other characters, but I stick with what I know and love.
There are only few (badass or not) enemy types that can be a bit of a headache, and soloing them results in sub-par and not very elegant strategies, but I can accept that even though I think it's pretty lame on the coding front, as there is no 'easy' or 'proper' way to do it. It always feels a bit like going back to play a beta of Doom back in 1883 or something. The majority of battles, regular or boss, is standard shooty shooty fare and is rather smooth. I don't like getting stuck on shrubbery and inch-high crap, which happens a lot, especially when facing the aforementioned total ass enemies, but if you keep that in mind and adapt, everything can be done.
One of the most important things is not to get too attached to your guns, as the vast majority of them won't last you very long if you level up quickly. No matter how good they are or how good you think they are, they just won't cause any damage after just a few level ups (and matching foes).
Yes, I have suggestions to make RE: SKILL TREE, but I think you should attempt to figure out what suits you and your playstyle best yourself. Try it out, money is pretty much free in Borderlands 2. Just sit down, go somewhere you like the scenery or whatnot and shoot things. Try maxing out one skill tree after thoroughly reading what each thingamabob does, or find fun combos and synergies that work for you. Do consider taking every ultimate ability for a spin, see how they work for you. Good luck.