Personally, I find them insulting. If I care enough about the show to be watching it in the first place I already know the back story. I don't need an entire episode to remind me. Maybe a brief "last time on" at the beginning of an episode could be helpful if there is a particular tricky or pivotal plot development. (Shows that I have found a brief recap helpful on include, but are not limited to: Battlestar Galactica, DS9, and Lost.)
I currently watch most of my t.v. shows from Hulu or Netflix, so when there is a recap episode I normally just skip it. Or read a book, clean, cook, etc. with the television on in the background. With the shows that I watch weekly (such as The Office) it would be immensely disappointing to tune in to a recap. All that anticipation would be for nothing.