Well, my hair goes down well past my nipples by at least a couple of inches, so obviously, I think highly of it. I originally grew my hair out due to my love of the heavy metal genre and its culture, but I now enjoy simply having it around and couldn't imagine having short hair again. The only downsides I can think of, is it takes a shitload of time to dry after having a shower, and on a windy day it will not stay out of your face. Both are trivial matters though.
Oh, and I have heard some guys find it harder to get jobs if they have long hair. That would be a downside as well, though I have no experience there.