Got my XCube II about a week, week and a half ago, and I'm loving the performance on regular mode. Haven't dived into temp control yet, but I'm eyeing some 316L SS wire and plan to start experimenting with it in the next couple weeks. Menus are really nice, too. Easy to navigate, and pretty self-explanatory.
The only thing I don't like about it is the lights. I keep mine turned off as they seem to produce a noticeable loss of battery life when turned on. Since this is my first 18650 mod, I picked up a couple different sets of the most popular batteries... 2 pairs of VTC4's from IMRBatteries , a pair of HG2's (came highly recommended by my local shop), and a pair of Samsung 25R greens both from the aforementioned local shop, and I've noticed decreased charge life on both the VTC4's and the HG6's when the lights are on, haven't tried it with the 25R's as I just got them yesterday and they went into use for the first time today, not sure if I'm gonna bother, logic and common sense say I'm gonna see the same thing... it IS an extra power draw when they're turned on, after all.
All that said, since I can turn the lights off, it's just a ding and not a serious fault against Smok. It's there if I want it, and not there if I don't. That's all I care about.
Upgrading the firmware can be a bit tricky at first. First, you must turn download on in the device menu, then you have to do the version check on the upgrade tool before you plug the mod in, THEN you have to hold down the fire button for the entire remainder of the process until it tells you that the upgrade is finished. Let go of that button even a split second and it's going to throw that error at you and you'll have to start over. Once you get the hang of it, though, it's pretty simple and quick.
All in all, it's a a solid mod for me so far and I've got no real complaints. Smok is hammering out firmware updates pretty quickly and, having been a PC tech for quite a number of years, this makes me much more forgiving in the face of the bugs that have been reported on functionality as it shows they are well in control of the situation and we need only be patient.
As a final note, right now I'm vaping on a .27 dual nichrome80 clapton setup in a Mutation X V4 with my XCube II and it's heaven. Not sure on the gauges used, the clapton wire was given to me at my local vape shop to try out the last time I went in to pick up juice, I only wrapped the coils from it. They recommended 6-8 wraps, and I used 7.
My recommendation... if you're curious about the XCube II, pull the trigger and bring one home. At worst, you get a secondary mod that anyone would be pretty happy to have on hand. At best, you love it as much as I do and end up wanting extras.