<< any sensitivity after treatment? >>
Oh yes. Its different for everyone, their degree of sensitivity, but everyone has some sensitivity after the first two or three bleeching/whitening gel treatments that use peroxide as their active ingredient. I recommend starting at around 10 - 12% Carbamide Peroxide, and then if you tolerate that well after a FEW treatments, then move up to the stronger stuff if you want (16, 18, 21%).
Do NOT use this stuff if your teeth and gums are not in good condition. If you have even minor gum irritation, untreated cavities, or gum disease, this can irritate the heck out of your gums and the nerve of your tooth. That's how I found out I had a cavity. The gel will not "harm" your tooth and nerve, but it will send you up the freaking wall if there is a broken spot on your enamel and the gel seeps in to the pulp and nerve of the tooth. Don't say I didn't warn you.
The gum irritation is usually minor if your gums are healthy. Many people report increased tooth sensitivity to temperature, especially after the first treatment, but it goes away after a few days. If you have REAL discomfort or sensitivity that doesn't seem to get better after a few days, STOP using it and consult with your dentist.
Most of this stuff on the market is the SAME active ingredient used by dentists, but they have the advantage of customizing your mouth trays so that it forms a good fit. This allows the gel to make contact with the tooth longer and helps minimize irritation to your gums. You want to bleach your teeth, not your gums. ;-)