Prepare yourself for a lot of replies, I'm guessing. The W126 chassis has a very passionate following, and is considered by many to be the finest long-range cruiser Mercedes-Benz ever built. As with all cars, there are some weak spots, but overall it's considered to be bulletproof.
Although I own a W124, I have looked at many 420 and 560SELs over the last few years and very nearly bought a couple. The only thing that stopped me is needing to sell my 400E to pay for the bigger car. I do love the 124's, and have decided to keep mine, but if I were doing it all over again and buying my 1st MB, I'd definitely get a W126 of some sort (but probably a 300SDL).