We installed rear ceiling speakers as part of a series of general upgrades we were making to our house. I got pretty good ones (BG Radia PD-6Ci's), and frankly, they don't add very much. Every once in a while, I'll notice the surround sound (good example: there's a trippy scene in the Doors movie, where the camera is circling around and the sound is as well), but otherwise, the front three are carrying most of the load. Also, there's a real directionality problem. You really notice when the sound is suddenly "up" instead of forward.

If I was going to do it over, I'd look at getting dipolar speakers, a few of which are available as in-ceiling versions. Allegedly these diffuse the sound better so you have less of the directionality issue. Unclear whether this really works.