If we are talking about unit longevity then the first eXcelon HU I ever bought, a KDC-X569 (2001 model year unit, I think?) is no longer owned by me but is still used daily by its current owner.
The unit that replaced the above, a KDC-X679 (2003 model year) is still alive and kicking in its current owner's '78 K-5 Blazer.
My current deck was purchased in Jan of '06 so wouldn't particularly be a good indicator of lifespan at this stage.