For the most part, I dislike them. I think their plots are usually really cheesy and uninteresting, with anthropomorphisation coming out the wazoo. I also think they have a terrible effect on ignorant people -- breed-specific, as Inga said, creating false myths about breed. On the one hand, they make breeds like the Lab and Beagle seem like born sweethearts (which we all know they are not) and on the other, they perpetrate the "tough villain" image of breeds like the Dobe and the Rottie, which Inga and many others like her fight to dispel.
Besides affecting the image of breeds, there's also the image of the dog in general as an animal that will come to you completely willing to lie on the hearth by the fire while you read a book, run around off-leash across grassy knolls, play Frisbee in the park... all things that many dogs can very well do, but only with training, and unfortunately the training part is the one most seem to miss.
I do watch them for one thing, though: the training. The shaping is incredible
in most of these movies, and I enjoy watching them and wondering exactly which cues are being given behind the camera, and how those cues were established.