Do a search on this site for "4-matic" or "TE" and you'll see many stories. Probably mostly mine. I had a 91 'TE 4-matic for 2.5 years, and drove it about 35K miles and it was endless problems. Despite that I loved the car. I have 2 friends that have the E 4-matic and a TE 4-matic from '90 and '92, respectively (both bought new), and they have only had the typical E class related problems, such as replacing the A/C control unit, valve seat replacement, replacing the oxygen sensor, vacuum system related problems, and so forth. If you get one, I strongly suggest getting one with a Mercedes Starmark warranty.
Oh, and due to the way the 4-matic engages, it did cause extra wear on the front tires. I rotated mine every 4K miles and they showed normal wear after about 20K miles when I traded the car.