Gem clone arrived on Thursday, my God it's good once you get it going.
I ordered the one with the dodgy 510 screw (I think the same tank with a fixed (extended) screw is available from some UK websites at a slightly increased price) but was able to fix it with a bit of insulating tape.
There's also a slight problem with the threads on the chimney section that means the base ring won't screw on straight. It happens with both the 510 and mech adapter rings, so I assume it's the chimney section at fault.
Took me a few goes to get it coiled at wicked properly, but once it goes, it really goes. 1.2 ohm, 13-16 watts, the taste is amazing.
I am getting very slight leaks and gurgling every time I fill it even though I close off the air flow and turn it upside down before opening it up again, but a quick upside down blow through soon sorts it. I'm not sure if it's just a thing with this clone or if it's because of the threading problem letting in a tiny amount of air. May even be my wicking, though it's perfect once it's filled and in use.
I've contacted fasttech to see if they'll send me a new chimney for it or if I'll have to return the whole thing. I hope they'll just send a new chimney since it takes so long for anything to come from them, but the clone itself performs brilliantly, though in my opinion it's using an awful lot of juice, I'm having to fill it every couple of hours at least.