I've played this solo, with the occasional splitscreen co-op, and loving it so far. Some of the equipment I have found are made for co-op play and they have me interested in trying it out online someday. I was really focused on the story up to level 20 but now by level 27 I could care less and I actually play with the sound off and read the text when I so please. This has not lowered my enjoyment of the game though! Still loving every minute of it. I even love stressing over choosing what equipment to sell or keep. Maybe try a few 'brief' plays with every character until you reach level 15 or so. Find out who you like the best. I'm playing as Zero, and as much as I enjoy playing as him I'd like to try someone else... Only I am too invested in my Zero character now.
So yea, try out all the characters before starting your first playthrough. See who's ability you would use the most and/or find most useful for your play style. (I usual play stealth in games that allow it...yet for some reason I do no use Zero's ability as much as I probably should)
And yea I realize I didn't really answer the question. I think.