You don't need a polar plot for a GPS to be useful for upwind/downwind training. Velocitek packages the concept nicely, but many existing GPS's can do the same thing (but on a smaller display), when used correctly.

Here's how I make my GPS + compass do the Velocitek thing in a matter of seconds:

Head into the wind or sail dead downwind to get the wind heading. Scroll the GPS map cursor 1000 miles away on this bearing. (My Magellan-based GPSs show the distance and heading on screen as I move the cursor.) Hold down GOTO(MARK) to create a waypoint there and GOTO the new waypoint. Display VMG, and this will give your true VMG to windward/leeward. The only caveats are that the wind may shift, and that the VMG is updated every second or two, which is really too frequent, especially downwind, where you would prefer averages over 15 to 30 seconds, IMHO, since good performance is much more dynamic than just selecting a heading, especially downwind.

Your GPS will now give you objective upwind/downwind feedback for training, and you will quickly learn what the boat feels like when you are sailing your best, so you won't need the GPS or compass to reproduce the performance when racing. And when you're in the fleet, nearby boats serve as excellent performance references.