I had an '88 for 2 years putting about 2,000 miles on it in 2005 and 5,500 miles in 2006 up until I had my accident in the July.
Did fine on the highway, sitting at about 120kph, between fuel stops. I think it was somewhere up around 52,000km on the clock when the insurers wrote it off.
Only issues I had were a bit of a thirst for oil on the hot French autoroutes on holiday, and the output shaft splines being worn out, so had to out in a grub screw to hold the front sprocket to the shaft.
Best change I made to the bike though was a Renazco Racing seat, from Boejangles on this fair forum
Once I'd broken it in after about 1000 miles, I had no problems whatsoever with monkey butt etc
Fitting aluminium panniers to H&B racks did affect the fuel consumption on the fast highway runs (down from around 55mpg (5.1l/100km) to about 45mpg or 6.3l/100km).
I've now got myself an Africa Twin, and obviously being a 750cc v-twin it's a lot smoother, but it's also about a third heavier, so not as useful on the tracks etc. Fuel consumption at the moment is only slightly worse on-island at 50mpg or so (5.65l/100km)