I recently went under the Brite Smile treatment. I was asked to brush my teeth with a special toothpaste before I went under the treatment.
The treatment is divided in three 20 minute parts. Every time a cycle ends, the gel on your teeth is reapplied and you start again. The first 20 minutes were perfectly fine. After that, I started feeling pain on one tooth. The same pain you get when your teeth get in touch with something really cold or really hot. The pain started moving to my other teeth. Pain came and went, it wasn't permanent but still it was uncomfortable. After the procedure I had the same kind of pain on my teeth for about 1 day. Then it went away.
It's worth it if you ask me, but be prepared to feel some pain, specially if you have sensitive teeth.