I started using my iPhone 3g with the official app, and didn't have too many problems, until I went to find a cache that was in a wooded area. I eventually found it, came home, and ordered an Etrex H straight away.
Once my money situation gets better, I would love to buy a dedicated GPSr that combines the paperless features of the iPhone with the accuracy of the Extrex H.
The mapping on the iPhone can be off sometimes, but using common sense, I usually do OK, but the battery is awful.
With the Etrex H, I seem to get to GZ a lot quicker, and if I am going anywhere near trees, the iPhone stays in my pocket. My only problem with the Etrex H is sitting down before I go caching and inputting all the co-ordinates by hand, then having to write down hints and other useful info before setting off.