The deepest coins I've ever dug were done with an old Garrett Deepseeker. They were in soil that was a real disgusting mixture of 10,000 year old glacial compounds. On top of that they had a salty brine in the soil and were found on the beaches at Tahuya State Park in Tahuya, Washington (State). Most were found from 10-13", in all-metal. The old Fishers though could get another inch over the old Garretts. The coins were mostly really rusted and corroded, sometimes partly broken up copper wheatbacks, but some were steel.
I have never seen another detector to date that could outdo the old Fishers and Garretts for all-out all-metal depth in really bad soil, but the 80's Compasses came close and so have some of the CZ Fishers. A person would be wasting their time in bringing along ANY Minelab vlf (this includes their multi-freq machines) or any OTHER brand detector too- to try to find anything any deeper than that in that very same soil. And yes, even though digging in that mess wasn't fun I would gladly do it again, even if the coins were as deep 2 feet.
Opinions about the TDI? Hmmm?
It's just a well-engineered (by Erik Foster himself) PI being sold with White's name on it with an operating method a little dissimilar in setting up and using, that's all. It is easier to use than a Garrett Infinium and a lot more practical than any other PI I've ever seen. Some people do like their various Minelab PI's but to me the TDI is a better deal and a whole lot cheaper too.
No, I wouldn't say it is difficult to learn, and it at least does have some reasonably effective discrimination plus a ground balance control to boot. It's a lot easier to use than a DFX or SE or Explorer II.
BTW, I see that on Metaldetectorreviews.net the last 3 out of 4 people have been fairly disenchanted and disgruntled with the new ML E-Trac.
Note: PI's ALWAYS go deeper in the ground than they do in the air.
LL