I've got a Cycleops Magneto that seems to be pretty good (the only one I've ever used so can't compare).
It was pretty easy to put together and set up.
I'd check out
EBay - mine was an ex-display for about half the usual price.
FWIW - it's useful having a computer with the sensor on the back wheel. You can then set targets for distance etc when you're using the turbo.
HTH