Hi Dave,
The GP 3500 is designed as a gold machine and as such should be viewed as an all-metal machine. In other words, you really SHOULD dig it all when running the GP.
But it does have a shallow ferrous reject circuit that more or less works, and a dual tone capability that affords some level of discrimination at depth. The dual tones are triggered more by signal strength than conductivity, but it often works that low conductive targets give a different tone than high conductive targets. Check out http://www.losttreasure.com/fieldtests/ArchiveView.cfm?ID=LT20050956 for details.
PI detectors are not for everyone, but if you had a place where you were sure good targets were out of range of a VLF that are worth going after a PI will get them at depths no VLF will match. Not even close. But the caveat of digging lots of junk and digging really deep holes remain, and so they are for specialized uses only. Like nugget detecting or beach detecting. The digging needs to be easy, or the finds of great potential value, or the area relatively junk free.
Last year I used a $600 Coiltek 27" x 21" DD coil on my GP 3500, and paid for it with the first nugget I found, which weighed over 2 ounces. I am now ordering a 40" x 20" Coiltek coil for use as a tow-behind coil for prospecting huge areas for eluvial gold and meteorites. Real long-shot type detecting but with large potential for a major find.
I'm convinced some form of PI detector will be what really advances detecting to the next stage. VLF units seem to have maxed out for depth of detection. PI units do have poor discrimination but so did the first VLF detectors, which were also all metal machines. That did not keep people from using them.
I really do not care about depth of detection too much when using VLF detectors. I really am willing to only dig so deep for a dime and plenty of jewelry is out there at shallow depths. But if a target is worth digging a deep hole and a lot of junk, like a big gold nugget or a large meteorite, and you want depths that are without question better than any VLF detector on the market, take a look at a PI detector. VLFs seem like surface skimmers by comparison.
Steve Herschbach