You can download Geocaches to the Nuvi and they will show up. I logged in to the the Garmin website and went to the Geocaching section. I was able to download Geocaches directly to the unit. No 3rd party software is required.
My Nuvi (250W) does not really guide you to Geocaches very well. I can route to them, I think they show up as a favorite. You do not get a compass arrow like handheld units. I tried zooming in to the max, around 5m, but it was still hard to find the caches. Once you are zoomed in that far, the map screen is basically blank.