So my Xperia X10i has had a Clear Coat on it since I got it, or almost. That makes it 2-3 years old now.
The CC has held up well. 3 years on, the CC is now visibly coming off at most of the corners and edges. It's still good and stuck on the phone overall, mind you, but it is clear it is losing its staying power. Eventually all the edges and corners will be loose.
The CC seems to have done it's job. I've dropped the phone a couple of times, and I don't see any scratches in the plastic.
So it seems to hold up over the long term. If I kept the handset, I'd probably cash in on the lifetime replacement guarantee. But I'll likely upgrade before then.