Personally, I hear mixed opinions, but my hairs really does thrive and grow well with weaves. I can wash my hair more often and moisturize and condition it often. Cornrows help keep moisture in my hair, so they just sit underneath the weave protected and well-moisturized. You get back what you put into it. With a lot of TLC, weave really can be good for your hair. Also, too tight corn-rows can be BAD BAD BAD!
I got a technique that left a lot of my hair out (pics in my album) and, although my hair did not suffer any damage during the time I had it in, The flat ironing I had to do to blend it in, would have eventually been bad. I think full sew-ins are better for me because there is no daily styling needed.
I have been a braid wearer for the most part, but that is just until I find someone that can do an A++++ job and give me a protective, natural-looking, flat-as-possible weave with straight hair. When I find that, I will weave-it-up as much as possible.
Now, on the other hand, I have heard of people having bad experiences with them. If you got the weave in too tight or unprofessionally that can be bad. Also, for me, synthetic hair dries my hair out BADLY. Some people do well with synthetic hair, but as long as use human, my hair does very well.