Before getting a new crown on an eye tooth, I wanted to have a good (and my first-ever in-office, and I was 49 at the time) bleaching. My teeth are quite white anyhow, so I did not have a lot of discoloration, but wanted to bleach to get a good color selected for the crown.
The Zoom process was rather uncomfortable, but tolerable. The worst pain came about 2 hours later, when I had a nasty headache. But pain was short lived and my teeth turned out a nice shade. No residual pain, although my teeth tend to be sensitive anyhow, but the Zoom process did not seem to make that worse.