I've logged something over 10,000 miles with my Garmin 605 since April 2008. The only problems I've had are forgetting to start it occasionally (I eventually changed it to auto-pause so I don't have to remember to stop/start it at stops) & once it threw a slight wobbler on a long flat section, suddenly plotting me a hundred yards off the road, several hundred feet in the air and travelling at nearly 80mph!

But then I've had glitches now and again with the more basic bike computers in the past too.
Battery life doesn't seem to be a problem so far - I've been using it for nearly 4 years and it has never been near run out after a long days ride (on for 8-10 hours, maybe more, continuously and still not "running on empty"), If I recall correctly it was showing about 1/2 charge at the end of an 80 mile ride last summer. On a multiday ride, especially if camping & not sure of a power supply, I carry a 5000mAh battery pack (Veho Pebble) that will charge the Garmin & my phone a couple of times.
Its been great for touring for logging where I've been & how many miles plus amount of ascent/decent. I don't do anything fancy, just download my riding to Garmin Training Center which has the added bonus of letting you view your route(s) in Google Earth. The other main use for the mapping is planning rides (usually on
bikehike.co.uk) which I then download to the Garmin and follow it as a "course" (basically a pink line on the map). I don't generally use the street-by-street navigation.
My only big critcism is the quality of manuals/ support from Garmin if you aren't doing racing. I've found
Frank Kinlan's Blog invaluable for filling in the gaps left by Garmin.
I've not got an 800 but it, or its successor, will almost certainly be my replacement for the 605. It is best to get the trail bundle if you want the OS maps as it is generally much cheaper that way round if you want extra maps than buying a different bundle & then the OS maps separately.
Regards
Rick.