Don't think it's metal. Ambit 3 peak has metal part too, yet its gps accuracy is still one of the best.
I'd say it's "OK" but not great. I did a ride with my 520 the other day and that was almost spot on the whole way round the ride, even getting the side of the road correct most of the way.
My 5 plus has me running through gardens or even the occasional house!
I'm going to try some of the other options - Galileo, GPS only etc and see how it works out.
For what I want it for it's great though.