I've ridden a few. Neil (Steptoe's the man to list their mechanical weaknesses) but from a rider's perspective:
I thought that the Paralever rear was an improvement over the Plain-Jane shaft of the K100/75's, the 16v model brakes were better too as was the use of the "S-type" forks.
I didn't like the fact that the fairing was moved forwards about 3-4 inches (it made it seem much heavier on the front end), the electric screen added weight, wasn't really necessary and was prone to failure. The rubber mounted pegs removed any 'feel' through your feet and the instruments moved from the handlebars to the fairing bracket and rubber mounted made them wobble about alarmingly.
They are seen as bit of a barge, limited options on tyres (Dunlop used to do a Radial but I don't think it's available any more, so it's bias-belted only). However, they can be hustled along pretty quickly and have enough 'real-world' performance to still be a quick and comfortable means of travel.
If you've never ridden an RT/LT BMW-Brick before, throw out all your preconceived notions, give it some miles to get accustomed. These are old-fashioned enough bikes to need a bit of planning in your ride. They don't flatter those who alter their line or speed mid-bend. Smooth is the key to getting the maximum from them and when you do get it right not only are they good fun but they can deceive lesser riders/machines.