I did the replacement a month or two ago on our Trek 850s -- got scarred and wish I had not. The cranks they provide as replacements are absolute worthless crap.

Huge Q Factor (visualize riding bareback on a Clydesdale), 170 arm length only, junk BB (creaky immediately).

The MC12's may have had some failures, but after riding them since 1995, I doubt mine were any more likely to fail than any other old crank.

I've since replaced with some fc443's from eBay that are a bit more palitable.

Shimano corporate corksoakers.