I've got three. One in the kitchen, one in the family room (for the kids) and one in the bedroom. These complement the mac mini hooked up to the projector in the living room.
I'm living off iTS TV shows (with a few OTA HD recordings recorded on the mini with a couple of EyeTVs) and overall it works really well.
The only thing I miss with my current setup is being able to watch a movie or TV Show on multiple TVs at once (the way I did when I had a central video feed to all my TVs).
Kevin