The strips are actually quite flexible and have a very good ability to adhere to the proximal contours of the teeth as long as there isn't too severe of a malocclussion(basically as long as there isn't too much buccal/lingual displacement of adjacent teeth). From the reports of my patients, for whom we've gone through about 50 of the professional white strip kits(roughly twice the carbamide peroxide concentration of the over the counter white strips) since February, there can be a little more difficulty with the adhesion of the mandibular strips over the maxillary strips. The rep from Proctor and Gamble who does my local marketing of Crest products suggested that we tell our patients to pucker their lips in the "fish face" manner to get better adhesion of the strip to the tooth.
The patients in my office who have bought the kit seem to love them and are quite happy with the results. Case in point, we tell our patients who buy the kits that if they aren't happy with the results that we'll refund thier money or put the cost towards either tray bleaching or light activated bleaching, we haven't written out 1 refund check yet!
Bottom line, this is a good, dependable product that works. It will take longer than either tray bleaching or light activated bleaching, but you'll get results with them.