So it's been a few months with my brass JGG and I wanted to share some of my experience. This is my first GG ( or a high end mech for that matter ), so consider me a GG noob and feel free to bash me if I say anything stupid here as I will mostly go over the negative observations - the positives were widely discussed here and I agree to most of them. Again, these are not "expert" opinions but rather impressions of a first time GG user.
The mod is beautiful and functional, that's for sure. Love the safety features! The button kept coming apart at first, until I really tightened it and I do find it very convenient: easy to lock/unlock, prevents mod from rolling. The downside is that I have to take the button down when I clean the mod and after doing this quite a few times I put several small scratches on the tube when tightening the ring with pliers. Same goes for firing pin - it is particularly tedious to get tight and I put a scratch on it while attaching it to the button. I wonder if this affects the voltage drop I am getting ( more on VD below ). I eventually realized that it's a good idea not to use pliers in direct contact and also should probably acquire replacement firing pins in case there's a chance it will improve VD.
Brass tarnishes like crazy, got so dark in just a couple of days that you can barely see any engravings on it. It gets much darker than some cheap brass mods that I have. Ultrasonic cleaner doesn't help much. Cape Cod cloths do help but doesn't bring it to original shine. So I basically stopped bothering and just let it go black. I also tried gaussboys magnets for button (really cool suggestion!) but reverted since I liked the tighter action of the spring.
Now, on to few more serious issues. I am a kayfun guy ( got two KFL, KFL Plus and KF Mini ) and first of all, the airflow control doesn't do anything for them. So I got stealth cap and found that you can't get the mini to sit flush with it. While mini + stealth cap would make a slick device in 18350 mode, I have to use the big cap just to have a flush fit. What's worse though, my KFL Plus which I just bought, got really stuck on a stealth cap, I spent few hours trying to take it off with no success - it is almost as if it got welded to the cap. That would be sort of ok ( I could "marry" the KFL to this particular cap and get myself another cap, but this is a KFL Plus with adjustable draw and it's got the draw screw on the bottom duh ). I am going to have to take the happy couple to a machine shop and hope that with the right set of tools they can be divorced as the tools I have at home just don't cut it.
And finally my biggest issue with the mod - the voltage drop. I noticed right away that it didn't hit as hard as my cheap brass mod ( that I don't even bother to properly maintain ), but only recently obtained a voltage meter and was able to look at some numbers. On a 1ohm kayfun coil I am getting 0.5-0.6v drop (under load of course ) on GG as opposed to 0.3v on my other brass mod. This is a significant and noticeable difference - like I said I felt it right away. I also have a brand new SS JGG and also decided to test VD on it and got 0.6v drop. Today I spent time cleaning threads with toothbrush and shining the contact points ( on a brass GG ) and still getting 0.5V drop at best.
I can't find much info on JGG voltage drop, so maybe 0.5V is considered to be in a "hitting hard" territory, but the fact that I am getting less drop on a cheap mod ( brass chi you clone ) that I spend no time maintaining ( just washing in ultrasonic once in a while ) I find upsetting. Right now I kind of ran out of ideas on improving the drop ( kick is not my thing as it doesn't provide enough power for sub-ohm stuff, I tried removing the bottom spring assembly and it didn't change the VD ). If the VD is to be improved, I believe it has to do with the contacts/threads, but as I described I did measure it on a brand new SS JGG with pristine contacts and VD was still big. If there are JGG owners who were not happy with voltage drop on an out-of-the box JGG and had to tweak it to get significant improvement, I would appreciate the advice. Or, conversely, if somebody measured out of the box voltage drop and found it far smaller, that'd be interesting too, would mean I am doing something wrong with the assembly.
Overall, I very much like the device, but I would only be in love if the VD could be improved.