I've owned 3.... I've done a full engine rebuild on one and complete restoration (minus engine) on another...
I've also done some big trips on them..
In a nut shell:
They are very very reliable. They keep on
ging with regular oil changes/valve clearance inspections.
They have their weakness like all bikes. Swap the fuel pump and carry a spare reg/rect and you're ready to go. Also keep the chain loose and keep your choke plungers clean and greased up.
They ARE NOT easy to work on. They are fiddley and have lots of plastic etc. WAY more aukward than an XT600 (I've also owned 2) You have to displace the rads to check valves, remove the exhaust to take off the clutch plate etc etc. Your AT is spread out all over your garge just for a 12 month service ! Not well thought out IMOH.
200-250 fuel range, trip counter , proper bash plate and robust design is also in its favour.. It's very hard to break one !!
Negatives are that its VERY HEAVY and Top heavy at that. Its no fun loaded up and the suspension from stock is pretty crap. The brakes are also crap too for what IS essentially a road bike. You can take it offroad and I have, but it's no fun !!
The stock seat is an arse killer and it's not happy doing more than 80mph on the highway with standard gearing.
The AT's are also expensive. I just sold my 45,000 miles tatty one for £2500. I don't think they're worth that kind of money but they do have a cult following which keeps prices high. You will also potentially end up spending a load of cash getting it right for you.
Getting a low mileage one which hasnt been around the world can also be tricky. Those which havent been abused go for £3000 plus which for a 10 year old plus bike is rediculous.
The XT600E has crap suspension and brakes too but its way cheaper, simpler and lighter. Personally, i'd have a look at the XT660R or the DR650SE !
All the best, Ted
P.S. I will probably end up getting another one at some point !