I've heard good things and some bad things about the units. I haven't used one on a boat but I have had an opportunity to play with one briefly. The developers of the thing really seem to be intune with what people want and I know they've been making changes to the device based on feedback...so some of what I say might be out of date.

The unit I saw displays only two digits of speed data. If you are at 5.4 knots - great. If you are at 15.4 knots - you only got to see "15". In order to establish VMG, you first much tell the unit the wind direction by clicking a button at a downwind point, then clicking a button after sailing to a point directly upwind. IMHO, this would be very difficult to do accurately and/or in a timely fashion before a start - especially on a lake where the wind shifts regularly and often dramatically.

As far as the build quality, the unit is bulletproof. I was very impressed with the construction. I wish they would come out with a unit that functions like the basic tactic unit with a magnetic compass instead of a GPS compass. A unit with angled displays and some better mounting options to provide some competition with tactic would be a market killer.