We had one (96 I think, not sure) araldited to the centre console in the AC12 I used to fly, and it wasn't much use. It kept the pax happy because they could see where we were.
I bought a GPS3Pilot a couple of years ago, and use it (as backup only) on most flights. My experience is much like the others here.
One observation: the database is WRONG in some important elements: it omits a variety of controlled airspace, so you need to look carefully at the map or you'll be getting an invitation to the Belgrano. ATZs are some strange sizes, that by an amazing coincidence happen to be US standard sizes (ie bigger than UK ones). So you fly in bigger circles round them, if you try to avoid. Garmin blame Jeppesen, Jeppesen blame Garmin, and nobody seems to have done anything to sort it.
To answer your questions:
1. Batteries: one of our Group has a yokemount colour GPS, and it eats batteries at an alarming rate. He plugs it into the cigarette lighter socket, so that's not a major issue. My 3Pilot goes about ten hours on a set of Duracells. It's not a good idea to use NiCd or NiMH if you're going to plug it into the cigarette lighter - there have been warnings about fire hazard (I don't know whether true or not).
2. I've not linked it with Flitestar, but I connect mine to the PC with Navbox Proplan, and it's dead easy to upload PLOGS and download tracks to stare at them on the screen. Bit depressing sometimes, looking at your track... I built my own lead (cost me a fiver in total) - you can buy them but when I looked the price was silly.
3. It's a nice reassurance. I wouldn't use it as a replacement for proper nav, because of the shortcomings above.
4. Yes, but in the 3Pilot you get either North American database or Rest of World database. I've not tried mine outside Europe, so can't comment more. It certainly has all the VORs and NDBs and airways reporting points I've ever asked it for. I suspect it's got a full IFR set, but I've not checked.
5. It does all I expected of it, and a bit more. It is stunningly accurate most of the time - when I land, it usually zooms in and shows me where I am relative to the runway centreline. I would guess it's good to within a few feet most of the time.
6. I don't do sailing, but took it with me when I went sailing with a friend. It told us we were on the water, and just outside the LTMA, but not anything about the buoys and channels. He had a similar one with a marine database,and that seemed a lot more useful
Negative points - that silly rocker switch to move the cursor - I just don't bother any more; the ease with which it can be switched to "simulator mode" and provide totally spurious information (just don't go near those menus!); you can't change batteries in flight if you're on your own - it has to come out of the cradle to do so; the database errors above; whenever you pass a waypoint, it zooms right out to show most of Europe and you have to press "Zoom In" the requisite number of times; if you "cut the corner" and miss a waypoint, it tries to get you to turn round and go back there - you have to "reset" the FPL; the screen's a bit small, but on the yoke I find it OK.
If I lost it, would I buy another one? If we weren't in the process of having a GNS430 fitted in G-UTSY, definitely.