While I own five iStick 20s and two iStick 30s (all of which are in use), my lone iStick 50 is, in my opinion, the pick of the Eleaf litter. Each newer and higher wattage release has improved upon its predecessors.
With the iStick 30, Eleaf addressed/corrected the main shortcomings of the 20W, namely, the 510 connection issues of cheap, soft threads and a fixed positive pin by making the threads stainless steel and the pin spring-loaded floating. Also, the voltage calibration was changed from mean to RMS. Other features added on the 30W included the flippable display and locking wattage selection, as well as the redesigned top cap "lip" to eliminate overhang with 22mm atties/tanks.
With the 50W, Eleaf has gone further, adding a second 2200mAh LiPo for a total of 4400mAh battery capacity. The case was redesigned with rounded sides on both ends for good ergonomics, the micro-USB charging port has been moved from the bottom plate to near the top of one side (to allow recharging with the unit standing up). The firing button is wider, curved, and doesn't rattle, and the up/down buttons and display were moved to the middle of the wider side face. Despite housing two batteries, the case remains about as compact (i.e., small) as it could be.
Significantly, the iStick 50W feels very solid. The 20W and 30W iSticks are nice PVs that perform well, but in the hand they feel a bit more like toys rather than instruments. The iStick 50W has the additional heft of that second 18650 LiPo, and the absence of rattling buttons gives it a more "industrial-quality" feel.
Will Eleaf come out with an even higher watt version iStick, say, 80 or 100 watts? I don't know, but if they do, I vote for ditching the shiny chromed top/bottom caps and buttons and replacing them with matte brushed stainless steel. I'm not particularly concerned with scratches, but all that shiny chrome reminds me of gaudy American gas-guzzler cars from the 1950s. I prefer a somewhat more sophisticated, "Euro-style" appearance. OK, I'm superficial---so sue me.
Performance of the iStick 50 is stellar. Instant firing and accurate regulation. No, the chipset/board doesn't provide true linear bucking for low-wattage, but it does use PWM to approximate a kind of step-down, and the output voltage signal is completely flat above the supplied battery voltage, so either the iStick 50 uses direct-DC for boosting or the implementation of PWM is especially well-filtered. Either way, what you set is what you get.
Along with a pair of iPV Mini 2 70Ws, the iStick 50 has quickly become my favorite, go-to PV. For the amount I paid---$45 shipped---what's not to like?