I bought this unit in lieu of a pump, since I hate frame mounted pumps, and the compact pumps won't fit in my under-seat pack. I got a flat last night, and got to try it out. (Some kind of reverse hubris at work here, I road two years without a pump and was flat free; but flatted two weeks after getting this inflator. Weird)
I discovered two things: i) always carry TWO CO2 cartridges ii) two CO2 cartridges may still not be enough.
It was night, and I could not find the pin hole leak in the tube to put the patch on. I used part of one canister trying in vain to find it. Fortunately the second hand store where I stopped to shop had a 700C tube sitting on the shelf! Unbelievable luck! However, the tire bead popped off during inflation, forcing me to deflate then inflate the tire again. I had just enough CO2 left in the second canister to put in about 40 psi. Had I had a mountain bike tube, I'd have been S.O.L.
Lesson learned; carry patches AND a spare tube.
Also, make sure you crank the cartridge tightly to the body of the unit, otherwise you will leak gas. (I was in a noisy area, and didn't realise it was slowly leaking at first)
The variable output valve was a blessing (the reason I bought this one, and not the cheaper unit)