The Paia Theremax is one of the best [b]kit[/b] Theremins available and is undoubtably the cheapest Theremin to feature Pitch, Volume, Velocity, and Gate/Trigger output voltages.
The Theremax has decent playability -- however not as good as the Moog Etherwave Standard.
Currently Moog Music is offering a rebate on the Etherwave Standard and you can get an Etherwave kit for little more than a Theremax. Unless you really want the kit-building experience and/or the control voltage outputs, the Etherwave (at the current pricing) seems to be a better value for the money.
The Etherwave kit's circuit board is already built and tested. Thus, the Etherwave is easier to build than the Theremax.