Heard from Mark at Precision Tandems in October of last year that he'd seen problems about ten years ago.
"When it comes to my experience with Cannondale...... it's not all that good with a history of frame cracks (20 of them out of 29 that I knew of but back in the late 90's and early 00's) that cannot be repaired."
We've only got about 3k miles on our 2010 Street Tandem, but it's certainly been rock solid.
Do take note of the
eccentric issue discussed on this forum a bit ago. Don't know if Cdale has dealt with it, but if you buy from Tandems East, it gets retrofitted without you even knowing about it.