I've had my Moog Etherwave theremin for about a week now and its absolutely the coolest thing I've ever played. It's also humbling. You need not only perfect pitch to play, but total body control - stance, breathing, these are all things that will affect how still you can keep your body so as to not lose the note.
They're really affordable too - I paid about $480 after taxes for my Moog. I say get one, I mean, it's not like you can't sell it if you don't like it. Be willing to dedicate a LOT of time to practicing though. I try to fit in an hour or two with it a day and my scales are coming along nicely. After scales I just throw on some tunes and do my best to play along. It's starting to sound less like a cat eating a baby in a blender, more like a theremin.
Oh yeah and you can run them through your bass rig, pedals and all. I use my old yorkville 400watt/115 combo for it and it's great.
Join the revolution!
It's a synthesizer that is controlled by waving your hands around two antennas. Even though it's not an effect, you'll find some discussion of it over on the fx forum. Also, while the Moog ones are really nice, the basic idea is easy to build cheaply with LDR's instead of those antennas, and that also means you can buy cheap homemade ones on Ebay all the time.
Theremins are awsome. 100% awsome. 
