I'm a jeweller and I've got one. As others have said, its only good for small items - eg. jewellery and dental work, but for such items its very good indeed. The H and O2 are produced in just the right combination to burn properly, but for most work this produces a flame that is just too hot. The gas mixture is passed through a single pipe into a separate chamber that contains a liquid, and from there to the torch via another single pipe. The liquid in the chamber is used to control the flame temperature: MEK is used for the "coolest" flame (can melt silver, gold, brass and iron), meths for a slightly hotter flame, and water for the hottest. The latter being able to melt platinum. The liquid can also contain a flux so that soldering can be done without having to apply flux manually.
The torch is about the size of a white-board marker, but a bit longer, and the nozzle is a hypodermic needle. The flame size is determined by the size of the nozzle used and ranges from one about 0.5mm long up to around 30mm long, the maximum size being limited by the amount of gas that can be generated and varies with the model of generator.
The advantages over using bottled gas are: no bottles that need to be refilled, no insurance problems with storing bottled gas, and no need to adjust the gas mixture; you just open the valve and light the flame. The main disadvantage is that only small items can be worked. I started with an air/propane torch, but the smallest flame was just too big for delicate work. I then got a small Oxy/propane set with bottled O2 which was fine for delicate work but always a problem with getting the O2 bottled refilled. The water torch is now used for most things.