My friend has 175k+/- miles on his 722.6 and it's still shifting like new.
AFAIK the fluid was only changed once, by him at about 150k miles or so. It did shift better after that.
Even the old MB trans usualy die between 200k-250k, if they are good, some much younger. My 300SD for example the PO rebuilt the trans before 150k miles.
So they seem like a fine transmission to me, last pretty long, shift real nice. The only downside is last I checked rebuilts were $3k.