Before I start talking about the car, it should be noted that while I've only had the car for 5 months, my dad owned it for 3 or 4 years, and my grandfather bought it around 1990, so I know the car pretty well.
Given this car's age, its performance is outstanding. The U.S. spec model is rated at ~8 seconds 0-60MPH, 140MPH top speed. I haven't tested the top speed, but I can tell you that mine easily keeps up with a Mustang GT to 100MPH. While not explosively fast from 0-~40mph, once it hits 2nd gear, it is a very, very fast car, especially given its 4200lb weight.
Handling is decent, although again, you have to keep it in perspective. Given the car's near-20ft. length and 2-ton+ weight, it handles very well.
Continuing on the subject of performance, the car is an outstanding sleeper. People don't seem to notice the "560SEL" badge, or at least they don't know what it means, because they tend to be real surprised when it turns out that that huge Mercedes is a lot faster than they thought it was.
In terms of build quality, the thing is built like a tank. Everything on the car, from the trunk lid to the window switches just feels solid. The doors don't click shut, they thunk shut. The term "bank vault" may be overused, but it's an accurate description of the car's build quality.
I find the interior to be very comfortable. The seats are firm, and there's a good reason for this - they are designed to keep you awake and energized on long trips. The dash layout is outstanding, the air conditioning produces colder air than my grandparent's 2003 S600 does, and the automatic temperature control is smart enough to not turn the heater on full blast before the engine has warmed up. In terms of features, it has a lot of what newer cars do - power front and rear seats, seat heaters in the front and the back, power windows, sunroof, etc. Unfortunately, it does not have a CD player (we're talking about a car from 1986 here!) and the stereo is horrible by today's standards, although it does all right considering that all it has to work with are 2 3.5" dash speakers and 2 6.5" rear speakers...
People don't give Mercedes enough credit for their tremendous engineering prowess, which is present everywhere in the car. Even the shape of the taillights has a purpose - the channels in the lens are there to blow snow and ice off of the lens. The door handles, too, are shaped the way they are for a reason. Their shape provides the maximum amount of leverage possible for opening the door after an accident. It's details like this that show just how much thought Mercedes puts into their cars.
My only real complaint is the gas mileage. The EPA rated it at 14/16, but with the way I drive, 12mpg is more realistic. This, combined with it's need for premium (because of the 9:1 compression ratio), makes for expensive fill ups.
Finally, there are a few potential problems with the 560SEL that you should be aware of if you're in the market for one. The 560's engine puts out just about as much power as the transmission can handle, so the transmissions tend to die around 100,000 miles. The timing chain should also be replaced around 100,000-150,000 miles as a precautionary measure because if it fails, you'll be needing a new engine. Finally, the 560SEL has a self-leveling rear suspension, which can be very expensive to fix if it needs it, so you should be sure that it has been maintained properly.