For its size the xp-40 is great pistol.
It has better range than a xp-20, but as opposed to the xp-20 it won't fit side pockets.
It has a much better capacity than the xp-20 and its range is a bit better (~1m).
You cannot fill it entirely, nor use up all the water.
The route of the water from the tank to the nozzle is a tube which goes to the end of the water reservoir. The main problem about this is that the water-intake tube must be submerged in order for water to be fired, otherwise, you will only be firing only air or mist. Therefore the angles at which the weapon can be pointed and fired are limited.
But these flaws wouldn't keep it from being one of the best small pistols, if it wasn't for its habit of breaking when you absolutely need it. Out of the 4 XP-40s I know, 3 have already broken (and wet there owners in the process).