I *strongly* advise anyone to think twice before deciding to get hair extensions. I had them done professionally (Great Lenghts) about two years ago, and they ended up completely breaking off almost all of my own, natural hair about an inch from my scalp (where the bonds were). My hair was broken at so many different lengths that it was impossible for me to even get a decent-looking haircut. In fact, I'm STILL in process of trying to grow my hair out to where it looks decent, because it had broken off at so many different lengths.
So there were two reasons they damaged my own hair so badly: 1) The extensions tangle very easily and are really difficult to comb out. They don't get the natural oils from your scalp, so they get dry really easily, and all that de-tangling/combing puts a lot of stress on the bonds and on your own, natural hair which the bonds are attached to. And 2) There is no easy or really gentle way to remove the extensions when it's time to replace them (which you have to have done every few months because they grow out and the bonds become visible), so when the extensions are removed, your own hair is easily damaged.
Also, the extensions come out really easily during both sex and combing. So while you might look great and have this nice, thick head of hair when you first get them done, that won't last long. I suggest anyone set their vanity aside and be thankful for the natural hair they already have before getting extensions. The people that do the extensions may tell you otherwise, but of course they have an ulterior motive because they get paid to do them!