I have had a Vista for a couple of months now, and have found a number of caches with it. The barometric altimeter is pretty good (there is a page on the GPS that can show trends in elevation or barometric pressure, along with the current elevation, ambient pressure, and pressure adjusted for altitude).
However, if you are going to rely on the barometric altimeter for elevation, then you will need to make sure you can get a good 3-D location fix for a fairly long time. The Vista will auto-calibrate the altimeter based on GPS elevation readings, but this takes a while...so don't be surprised if the first time you switch to the barometric altimeter that it seems to be way off. It will tend to get better the longer the GPS is turned on. You can manually calibrate the altimeter if needed, but I find that the automatic calibration is fine.
A friend of mine has the Garmin V, and while it is very cool (especially since it can do routing) I really like my Vista. It is compact, doesn't hit the batteries too hard (12 hours on a set of AA's, and I just got a NiMH charger at Sam's Club with four 1850mAh AA's for $30), and I don't think twice about taking it out in the rain. The compass is wonderful for homing in on caches. Overall, it's pretty rugged, but I'd recommend getting a cover for it (they make ones that allow you to use the GPS with the cover on) to save the display from scratches.
The 24MB of map data is pretty nice, as well. It is enough to store about five major cities and all of the smaller cities within 60 miles of them, in my experience (your mileage may vary, but I can get Sacramento, Denver, Houston, Tampa, and Orlando all at once, plus a lot of outlying areas).
The only real downside of the Vista is that it does not have a big antenna (which I only notice because my friend with the Garmin V seems to get better position fixes under tree cover). I don't really notice this, though (it causes an increase in error, but not by more than 10-20 feet under heavy tree cover, and out in the open, it's right on). If you are particularly paranoid about being able to get a good signal in a car or something, there are repeating antennas you can buy that will put a stronger signal next to your GPS anyway.
Overall, I'm quite happy with mine. I spent $350 on it at Best Buy, and have probably put 50 or 60 hours on it already.