Just started Arch last night after hitting 83 and thought I would add a few comments....at 83 each successful item was bringing in 11k experience.
There are usually more than 1 item at a site (Thandol Span digsite had 3 last night). So keep digging until the shovel no longer works in that area or the marker on your map is gone.
For those much further along at what levels did the items start getting useful (pets/mounts/weapons/gear)? I would guess gear not till 450+ but how about the other stuff?