just a note... I have the eTrex Legend (the model right before the Vista) and it doesn't get very good reception in my XJ. I usually leave it on the dashboard until I need to look at it, using my left hand to hold it near the window. In a soft-top you'll probably get much better reception, though.
Another thing to consider, is that under good tree cover, the small GPS units (ie, with small antennas) like the eTrex don't perform as well. Outside with a clear view of the sky, they're awesome, though. The WAAS feature lets you know the accuracy of your position, it'll get you within 15 feet of the exact position if you have a clear view of the sky.
my next GPS will be one with a much bigger antenna, or at least one that I can hook up to an external antenna and can put it on my roof. That's what we do in my Civil Air Patrol mobile command truck.