I found the DeLorme PN-40 to be a great device that is waterproof, expandable, and rugged. The Topo 8.0 software is very powerful (though a bit intensive) and the maps are among the best I've seen. DeLorme is slowly beginning to support Mac users, which is a bonus.
If you watch for it, the PN-40 will often be on sale for under $300 at Amazon.com.
I've tried the Garmin 60CSx, but did not find it to be as Mac-friendly as it was supposed to be, and the very first cache I hid seemed to have wacky coordinates in spite of my sampling of the site numerous times. I also could not get the unit to remain stable when I was searching with the compass. DeLorme's compass is superior and more accurate in my opinion.
I also found DeLorme customer service to be quite responsive. This is simply my experience and I'm sure many others have been very pleased with their units. It's very subjective and a very personal choice.