I think that guys like naturally long hair rather than hair extentions. But, for someone who loves hair extensions or need them in this case - if done properly they can be very attractive. Some girls I know get extensions that either don't match their hair color or looks too over done and fake. On the other hand, there are some girls who get it done through a professional salon, which makes the hair seem real and no one would bother guessing whether its fake or real. So, to answer your question - Yes guys would like hair extensions on a girl if its done properly. But, as oppose to naturally long hair vs. extensions - most guys would go with natural.
I personally have never had extensions because I already have long hair, but I did go with one of my friends to get her extensions done. Firstly, she told me it hurts like a bitch, and secondly - her hair has been damaged from getting extensions over some time now. To me, it sounds like a bad idea and wouldn't recommend it to anyone. But, it really depends on you.
I'm the kind of guy who's into girls who rely as little on cosmetic products as possible. Hair extensions, jewelry, tattoos piercings, dyed hair & colored contact lenses are all things I don't like.
And one other thing that I can't stand is white girls with dark tans and bleach blonde hair so that their hair is lighter than the skin. That is gross! Especially when you consider how that tan is gonna royally screw up their skin in a few years.
My cousin's a hair dresser and she gave me extensions last year for prom. They didn't add length or anything, it was just because my hair is really straight and fine, so the extensions added volume.
I wouldn't really say they damaged my hair, but they were sewn in so I had to be careful about how I washed and brushed my hair. I'm pretty low-maintenance, so it kind of sucked, but my hair looked better with the extensions.
When I go out I'll sometimes wear hair extensions to add a little length and volume. My boyfriend at first thought it was kind of weird but now he couldn't care less. I don't think any guy really likes when a girl who uses a lot of makeup, hair extensions/wigs, etc. but if he likes you enough he'll get over it.
Hair extensions are really hard on hair, but they look good. My mom works at a salon and the best out there are called Micro Point I'm pretty sure. But they pull on your roots so you have to be really gentle.