for what it's worth, I think they're terrible. Every apartment I've been in that has them gets them removed IMMEDIATELY and put into storage until I move out!
Henry's right - the tracks are never clean, you can almost never dust/vacuum properly around the harware in the floor, and so on and so on...
How deep is the opening for the bed going to be? I don't know how practical it would be, but that door style that is often used on armoires to slide the doors back into the cabinet would be an interesting alternative - kind of a cross between a regular door and a pocket door.
For what it's worth, the place near my home that manufacturers Murphy beds has several models that don't use doors at all, but instead have panelling on the base that blends nicely into the wall. Pretty spiffy, actually!