One for home and one for work. I've been using them for years. I tried the 4000, but I didn't like the feel of the space bar which is the key you use the most by far.
Yep, I try to keep an extra one or two in the basement myself in case my main one goes bork. I think the feel is far superior to the 4000. I also put down the $$$ for the Kinesis and, though I like it a lot in many ways, it is a bit more than I need. I'm doing this primarily for comfort and speed, not out of CTS-induced necessity.