This is my first 18650 with built in charger. I’ve seen them around for a while, but never went and got one. Wellp, the opportunity presented itself recently with the Nitefox promo. 
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Nice cell, and convenient to use with its built-in usb charging.
It’s rated 2600mAH and indeed clocked in at 2605mAH on my Opus '3100, and measured 190mΩ which is reasonable for a protected cell. I observed and confirmed with Nitefox is that it’s limited to 5A or so, so it’ll work in most lights, but I wouldn’t use it in any light that also doubles as a curling-iron, ie, anything that draws a fiendish amount of current. Most normal flashlights (that draw 3A or less) will be fine.
I had already partially quick-charged it, then plugged it in via its own charging port, and it did finish topping off the cell. Light went from red to green and everything. If you want to use a light with a decent 18650, and don’t want to have to drag around a charger as well, this’ll do perfectly.
Be aware that this cell is quite long, with the protection circuit and charging circuit, so space might be tight, but I haven’t encountered any of my lights yet that have a problem with it. If you can fit a protected Panny-B, you should be able to fit this one as well.
Bottom line, I’d say this’d be a great cell to use in any light that doesn’t have its own charging port. By just carrying a cable, you could plug it into anything that has a usb port and top it off. Pretty nifty.