A Navik steered my Ranger 29 across to Hawaii in 1996 and worked great.
Another Navik was doing great in July 2008, but the underwater paddle exploded sometime in the middle of the night while I had the windvane "unhooked" and an autopilot was driving. It had driven me along just fine for the first 9 days of the SHTP in my Santa Cruz 27.
The Navik is really light, but it's not made any more so if you want one you'll have to buy a used one or score a deal on one that someone never took out of the box. It's getting really hard (read: next to impossible) to buy replacement parts. Also, the one real drawback to the Navik is that it's essentially impossible to pull the pendulum oar out of the water while the boat is moving. "impossible" might be exaggerating, but *Very Risky* to fingers and wrists and hands is no exaggeration at all. There's a reason I left the oar in the water that night.
I gotta agree with most of the above posts, unless you have a small boat, go with the Monitor.
1968 Selmer Series 9 B-flat and A clarinets
1962 Buesher "Aristocrat" tenor saxophone
Piper One Design 24, Hull #35; "Alpha"