I got one a few weeks ago, took some fiddling to download the maps/charts, as manual is not particularly overfull of advice, but Garmin UK were outstanding on telephone help line. I must have racked up at least 20 calls over a couple of weeks & unfailing courtesy was the order of the day. One tip, do not try to down load maps/charts to BOTH the internal memory AND the datacard. It just dont work. I now have 2 datacards, one for maps, one for charts. The other thing was you have to "change" the voice commands, otherwise you dont actually get any!
Overall, not bad, although the "find address" & subsequent "go to" could be less fiddly. Not used it sailing yet, as boat laid up for the winter but cant find any major problems yet.
Only real downside if being used as backup onboard is lack of water protection. It is necessarily small & bet a dollar it gets dropped in the bilges at some stage of the game!
Cheers
I have been using an iQue 3600 since they were released and have found it an invaluable tool for the car, but not really that good for boating.
I tend to use my laptop with admiralty charts in memory map for full on chart plotting.
If you have not yet bought one you might find a better use for your money such as a garmin 176 second hand with bluechart for a lot less money than the 3600. Plus you buy the ique and you will still need to pay extra for the charts as it only comes with the road mapping.