I had Great Lengths extensions (the ones that bond to your head, strand by strand). I liked the way they looked at first, but they get annoying really fast. When your natural hair falls out, but it can't go anywhere because it's glued to the extension, you end up getting a slight dreadlock effect after a few months. I think you can go to a salon and have them "cleaned up" but after the 4 month mark I was ready to have them taken out! The most annoying things for me were: 1) not being able to properly brush my hair - they give you a different hair brush and you never really feel like you are brushing your hair properly, since the brush doesn't touch your scalp. 2) You can't wear your hair up without the bonds being seen around the sides and back of your head. 3) You really have to fully style your hair every day. I'm used to just throwing my hair up in a ponytail every once in awhile to give it a break, and I couldn't do that with extensions. When you wake up in the morning, your hair looks gross because it's all flat and the extensions are really obvious.
Having said all of that, I did love the way they looked when I styled them properly. I got mine done in London Ontario and it was about $600. I've considered getting them again but I think I would look into the clip-on ones instead...having extensions for 4-6 months straight is a big commitment and a lot of work!