I am a big fan of the 123 wagon myself. If I were to get one, it would be an alternative to a minivan, SUV, or pickup truck, or an "upgrade" for my wife's 95 Subaru wagon. She doesn't like the styling of the 123 wagons, but I think they are roomier than the 124 wagon, and much more affordable than the current E class wagon.
I must agree with Benzmac on the collectibility question; a coupe or convertible would likely be more desirable in a couple decades, but for the utility factor, my needs run more towards hauling lots of luggage and cargo.
Nice thing about the 123's, they are cheap and reliable. I'd seriously consider trading my '84 SD on a 123 wagon in similar condition. To me, the 123 wagon is not the prettiest car MB ever built, but its simplicity and utility are hard to beat.
On a lighter note, I once heard a comedian compare a Mercedes station wagon with a Rolex clock radio...