I drink coffee regularly and also my teeth are just naturally more yellow than most. I did the 90 minute session today. It was easy to schedule online.
The process was explained well and I felt comfortable the whole time. I didn't have any gum or teeth sensitivity afterwards. My lips were a little tired afterwards from keeping them stretched on the device that holds them open for treatment, but that's not surprising after 90 minutes! There is a TV to watch so you are not too bored. During the 90 minute session, you do four 20 minute treatments, with a couple of minutes break in between to rinse with water and reapply.
I can tell my teeth are whiter, but it's not a very drastic improvement like other pictures show. I think I have difficult teeth. I'm planning to go back for a 60 minute session and I hope I can get a few shades whiter.