I've used mine a bit (much more useful now I have a chart card...) but the range seems pretty limited. It's very much line of sight too - with the boat parked up by my house I usually don't see an AIS return from boats in the outer harbour which is only maybe two or three miles away though with a small ridge line in the way - but VHF reception would be 100% at the same position.
I'm not sure what to make of this - the AIS in the office has a range of about 50 miles but with a high tower mounted aerial so it must be a limitation of the aerial or receiver I think.
The other really annoying thing is that when you get close to another boat (less than about 1/2 mile) you get a collision alarm on the Garmin 556s and though you can switch it off, it insists on coming back on again every time you switch the Garmin back on, and every time you get close to a boat
I guess it is a safety feature but it's a PITA not being able to turn it off as on a RIB you regularly pass close enough to other vessels to trip the alarm without being any risk so it is stupid.