I've got 291,000km (180k miles) on my W123 280E and apart from the occasional abrupt shift when cold, the tranny is fine.
I gave it a fluid and filter change which improved shifting no end about 2,000km ago: I'm pretty sure that the previous owner neglected it. It had the wrong filter fitted, for a start, and the fluid was dirty.
Next step is to get the linkage adjusted ... there's a little reluctance to shift which is probably down to the controls rather than the tranny itself.
Having said all that, this is probably an extremely dangerous thread. Everyone who posts here will probably suffer transmission failure in the next few days!