Personally, I never notice it. Hair is simply so far down on my list that I only notice it if it changes radically, like a primary colour or gets shaved off completely. I don't think I know any guys who notice it at all.
I notice when a girl does her hair and I really really really like it! ( Yeah that's three reallies lmao =P) It shows that a girl likes taking care of herself and that she wants to impress me and is willing to take the time to do so. It doesn't have to be anything to grandiose, just something that works for her and shows that she puts effort into how she looks =)
My guy always notices my hair lol I don't know why, his favorite style is when I let my hair do the natural beachy wave thing, and his favorite color is either natural, or this deep auburn lol whatever the reason, he is always letting me know if it looks good, I love it haha
yeah a guy in my college class notices if I have my hair up or down, a different colour, whether its been straightened or not or if I've had a cut when some of the girls ddont even notice. I think guys pay more attension than we give them credit for haha