Ok, well it's blazing hot outside today, and I really don't feel like going chasing clad, so I thought I'd put in my 2 cents on this...
Now, these observations are based on me being primarily a coin hunter, but they may be helpful to a 66 used in general.
I've been using the old-school Fishers since 1986, and I have (with all earnestness and humility) amassed quite a collection of great coin finds with both the 65 and 66.
BB, in response to your comment about the 65 maybe having a little more depth - I'm not
completely certain on that, as I am not hunting in the same places/conditions with my 66 as I did with my 65 - but I can tell you that I got
freaky depth on coins with the 65... oddly enough, the deepest coin that I actually measured (I had a big@ss trowel with a ruler etched into it) was a silver nickel at exactly 10"... I do agree with you about dimes, though, however there was one morning that I hunted after a very heavy rain (I was actually recovering coins in water-filled plugs) in which I found
6 Barber dimes on an old school field that gave me nothing more than a sweet, tiny scratch, and it was only due to
experience (and the fact that I was just on
fire back then
) that I was able to find them. I've air-tested my 66 recently and got as much as 13-14" on a silver half.
As far as an overall consensus on it as a
coin hunting machine, it will hunt deep - the only thing (at least for me, anyway) is that I cannot say with certainty that it's the best machine for hunting in urban, trashy areas. If you're not used to it, or don't have the patience for all the noise it can make, it has the capacity to drive you
insane. I still reach my threshold for it's noisiness at times, and just have to switch 'er off and find another spot to hunt.
It does indeed have an affinity to iron, and will sniff it out deep. It also has an affinity to the tiniest of objects, which can be a plus, however, you can adjust it to ignore smaller items and focus more on coins. Lately I have been hunting with #1 on 7 and #2 on 10, and I just switch over to eliminate Zlincolns... to me, they are just absolutely worthless, unless you recover it early, or just eyeball it. Plus, even though it's still a coin, I'd rather spend the time recovering a dime or a quarter. They'll always be there if I'm ever that bored, but, I doubt that'll happen.
It also absolutely
loves silver, and due to that fact, and that I'm primarily a silver hunter anyway, I don't mind using that combination of discrimination, as even on a setting of 10 it will
not discriminate out most any reasonably-sized silver object.
One peculiar idiosyncrasy it has is that at a certain depth (i.e deep), even discriminated objects will occasionally hit as a good target. This can work both ways for you. I have recovered a nice gold crucifix way down deep while hunting at 7. Most times, however, what I end up digging is a small bit of foil or aluminum that is between 4-6", and I don't really mind that, as I will occasionally find good stuff that way too, like the crucifix. I basically use the pinpoint to eliminate the smaller, shallower objects.
[Regarding pinpointing - this is one of the 66's BEST features. A nice feature of it's pinpointing abilities is that you can "shrink" the pinpoint signal down by repeatedly toggling while lowering the coil to the ground. I can usually hit my coin dead-center with my screwdriver - unless the coin is sidways - and in that instance, I usually end up going down to my elbows just to finally notice the Lincoln penny on the side of my plug about 4" down.
You can also determine depth with some degree of accuracy using pinpointing, as well as eliminate larger objects by lifting the coil.]
Hunting for gold (jewelry) with the 66 can honestly be tricky and frustrating. Being an analog machine with virtually no ID (built-in), once you've accepted nickel-conducting objects, you're digging almost everything from small bits of foil, can-slaw, screwcaps, and aluminum in general, at any depth. I have not been able to successfully find a dual-disc setting that allows me to weed out the junk. Please, if anyone has any insight on that, let me in on it.
As far as sensitivity settings go, I have not seen how adjusting the sens. affects depth to any viable degree. The main thing that adjusting the sens. settings affects is the size of the objects it's seeing. Most gold jewelry objects are generally at least
somewhat coin-sized, therefore finding a sens. setting that eliminates most smaller-than-those objects is advantageous when searching for gold. Also, most of the time I'm a "scrubber" - running the coil right on the ground, but I have found that in high-trash areas, that running the coil a couple of inches off the ground will help eliminate some smaller trash items. Nothing it seems, however, helps eliminate that @%#$@%# pencil erasers.
There are a couple of things that I just plain
don't care for about the 66, and those are - its weight, and its tendency toward interference when near power lines and some types of buildings (and, just about any other metal detector or cell phone made - my cell phone makes it go nuts, so I usually don't carry it with me while hunting unless absolutely necessary). I was hunting some old easement in a downtown suburban office area yesterday, and in one spot it was talking to me so bad that I had to leave. It will usually remain quiet in these areas while you're
swinging it, but when you stop to dig, it will buzz like a swarm of angry bees, and is just irritating beyond
belief.
Regarding it's weight, well, it's just a
beast. I'm not sure what the specific weight is, but compared to other machines, it's built like a brick $hithouse, and will wear you out fast if you're not used to it. I can swing it for several hours before I get fatigued, but I still do get there. And unfortunately, given the way it's constructed, there really isn't any pragmatic way of rigging a way to hip-mount it.
So, there's a few of my gleanings from 20+ years of experience with the 126_-X series... I hope it is of some help.
BB, please, don't hesitate to agree/disagree/comment on any of the things I hit on in this post, as it is rare that I have access to anyone else with as much experience (and success) as you.
Long Live the 1266! 
