I tried the Custom Maps option on my GPSMAP 62 yesterday. I took a map of Lebanon Hills (It is a Park in Eagan, MN), scanned it in and synced it up to the map on Google Earth. I was skeptical that it would work, but I have to say it was very accurate.
It was very nice to be able to see where I was on the map. Has anyone else tried this? I thought it was pretty cool. Instructions

I use this a lot. I grabbed screen shots and created jpg files from the Superior Hiking Trail website and have them in my 62st for reference when hiking there. Other maps have come in handy when walking out to the ice caves in Apostle Islands National Lakeshore, hiking in Glacier NP, skiing Baker Park Reserve, showing the borders of the Pagami Creek Fire as we paddled, etc. Really cool and quite accurate if you take the time to do a good georeferencing.