This is my case study about geocaching with a Mac. I've only been doing geocaching a few weeks, so I'm certainly not an expert yet, but here's at least one way to make it work.
First, my hardware is as follows: Titanium PowerBook running OS X 10.3 (Panther), Garmin ETrex Legend, and Palm Tungsten T3 handheld. I use a Keyspan Serial-USB adapter to connect the Garmin's serial cable to the Mac. The Palm connects via USB.
For software, I just got MacGPSPro, which I use to transfer waypoints, routes, track logs, etc. between the Mac and the Garmin. MacGPSPro can use any map you can get onto your Mac clipboard, which means I can download and use PDF-format maps for local county parks. Once the map is calibrated for use with MacGPSPro, the program displays waypoints from geocaching.com loc files on the map.
I had considered other Mac-GPS software but after reading the testimonials on the MacGPSPro website ("I got down on my knees and thanked god for this software!) I was sold.
I use the Palm to store loc files so I don't have to carry printed cache pages. I use two pieces of software: CacheMate runs on the Palm and is essentially a palm database application. I can search for a cache by name or by proximity to a set of coordinates. CacheMate also allows me to enter logs, so I don't have to try to remember what we took/left, etc. when we get home. I use a Mac application called MacCMConvert to convert loc files to pdb (palm) files that can be downloaded to the handheld.
Like all things Macintosh, all this stuff was a piece of cake to get working. I'm sure there are other options for software out there, but I'm very pleased with how these applications work.
I haven't yet become a premium member, but I plan to do so as having the full cache listings on the Palm will be very helpful (CacheMate will download gpx files too.)