It depends. Some guys can look really, really good with long hair (particularly if you look after it - long greasy hair = eew), and some guys just can't get away with it. Usually it looks better tied back (though not always, mainly with dark hair) and thin hair doesn't tend to look good. Much past shoulder blades and it starts getting silly, unless you have dreads.
Bit of a wishy washy reply, but it all depends on the guy, really, and those are just my personal preferences.
(That's from a girl's point of view, folks)