I love these devices. We use them to control a pair of hard-to-reach lamps and my only regret is that we didn't get them sooner. The whole system is really well designed: the remote is simple and clear, the controllers don't block the lower plug on the wall outlet, and everything is clearly labeled. Each package comes set to a particular channel, so you can get more than one set without them interfering with each other if you are careful to check the labeling first.
My only complaints are that we really could use a second remote, the remote itself has a stupid keychain attachment that is molded on (we removed ours with a Dremmel...why would anyone want to attach this thing to their keys?). The battery is also a pain to change since you have to unscrew the remote to do it, but it's one of those 12V cells that last forever so you won't be doing that very often (though prepare to pay when that time comes).
If you aren't afraid of a soldering iron you can easily change the channel assignments. Just look for the contacts labeled A0 through A3 and think in binary. Of course doing this will void your warranty, but it's a fast way to get multiple remotes for the same outlets, and multiple outlets controlled by the same selector. (There are 16 possible channels, making the "buy two sets and hope they match" approach impractical.)
UPDATE: December, 2010
The latest version of this product (as of Dec 2010) has a much smaller remote powered by a button cell battery and a small, telescoping antenna. The battery is much easier to change (there's a slide-out holder on the bottom of the remote) though I found that at first the battery wasn't making good contact and I had to re-seat it a couple of times. The antenna really does make a difference: a couple of our plugs are buried out of sight, and without the antenna extended you have to be within 6 feet to switch them on and off. With the antenna up the range is nearly double.
The older and newer versions of this equipment are not compatible with one another, so even if you get sets with the same channel assignment, the older remote won't work on the newer plugs, and visa-versa. This is not a big deal but it's something to be aware of.