I don't have a dual-field, but have the earlier single-field Surfmaster Pro. I have a couple of friends with the DF's, they love them.
If you've never hunted with a PI machine (any brand), get ready for a shock. At the beach, it seems as though there are two enemies...hairpins and nails. PI machines, as you said, don't discriminate...period. They hit on everything. That's why some folks prefer a discrimination machine like the Excals that can help eliminate those pests.
Once you get a feel for the PI's, you can fairly easily discern bobby pins and nails from good targets. BP's will actually give you a quick double-hit, but you gotta be listening or you'll think it's one sound. Nails definitely scratch as opposed to the solid hit from a coin or ring as the field hits the edges. I know it sounds like voodoo, but it works.
One more comment about PI's. Some folks take them outta the box and start air-testing. They are often disappointed before they ever hit the beach. Generally speaking, a PI can't find a dime at two inches in the air, but will send you digging to China after the tiny gold back of an ear ring in the surf. I like 'em.
