Have you ever had your teeth professionally whitened at the dentist office? How was it?

Is there anything I should be aware of before having it done?
--------------------
Many dentists are of the opinion that professionally dispensed take-home whitening kits can produce the best results over the long haul. Take-home kits incorporate an easy-to-use lower-concentration peroxide gel that remains on the teeth for an hour or longer (sometimes overnight). The lower the peroxide percentage, the longer it may safely remain on the teeth. The gel is applied to the teeth using custom-made bleaching trays that resemble mouth guards. Take-home teeth whitening kit cost: $100 to $400
Source