My experience with the X7s has been inconsistent. On the one hand, they produce the best flavor of any method I've ever used, and I've tried just about everything. Flavor is even better than atty-dripping. Throat hit is solid with real impact, and vapor production is good. On the other hand, the draw is so smooth that it's almost airy. I have to be a little careful about drawing very evenly, without taking too hard a drag.
I like the blue LEDs, which provide instant visual confirmation that the carto is indeed working before even taking the drag. I like the tubes, which are strong, flexible, thick plastic that doesn't crack or break (the same unbreakable tubes are used in Ultimate CE2 Clearomizers). I like the bottom coil, and the absence of rope-like wicks. I like being able to see the juice, so I know how much is left. I even prefer the smaller 1.3ml capacity to the larger capacities of carto tanks. And X7s perform beautifully from 3.7V up to about 4.5V.
HealthCabin states that most standard 510 drip tips will fit, but they don't. Period. The inner draw tube inside the X7 is too big to fit into the inner hole on the drip tips, but that hardly matters, since the outer diameter of a 510 drip tip is too big for the X7's outer tube and won't even start to go on. The only drip tips I've found that work are the soft silica gel 510 drip tips that HealthCabin sells, which I happen to like, so that's no big deal to me.
The leaking/pressurization problem is mysterious. I fill my X7s with a syringe (some folks hate that, but I don't mind at all) and put a condom on the connector while filling. Some of my X7s seal perfectly and don't leak at all. Others leak right from the start. Still others work for one or two fillings, then suddenly leak. And when they leak, it's awful. We're not talking about a minor leakage here---when one of mine starts leaking, 90% of the juice is gone and wasted within about half an hour. That's a real PITA when it happens overnight, and I pick up the PV the next day to find it covered with juice. What causes this siphoning effect is a mystery to me.
Blowing them out into a kleenex after filling to clear the draw tubes seems to help prevent leaking. I waste .1-.2ml of the fill that way, but it's worth it. At least that's what I'm experimenting with now. So far, so good. If that turns out to be a dependable fix for the leaking problem, I'll buy another 25 X7s to go with the 25 I have now. If not, however, and they continue to leak sporadically and randomly, I'll eventually just throw them all out and stick with poly-filled Resurrectors, dual coils, and Boge LRs and SRs (which I continue to use anyway).
I gave up on CE2s eight months ago as hopelessly flawed products, and I routinely curse Royal Smokers and MicroCigs as companies that just don't give a damn, but I was willing to give them a second chance with Micromizers. So far, my jury is still out. X7s are the greatest thing since dual coils when they don't leak, but when they do, it's just more of the same old crap from Royal Smokers.