From what I have seen, battery life is disappointing for most of the UMPC's which are currently available.
However, (despite their cost) I have been rather impressed with the Sony UX series of UMPC's, they have reasonable spec's good battery life and they look to be very nice (and did occur to me as being a possibly nice tool for photography) - the problem is that they are expensive, and I am by no means convinced that they are more worthwhile either than buying a load of high capacity memory cards, or, buying an ultra portable laptop and some portable external storage.
Of course, in the end, the question of if a particular piece of computing kit is for you can be answered by first working out what you want, then looking for the tool which fits best your requirements, so if you want an expensive portable machine which is pretty much only good for use either for tethered shooting, or when hooked up to a big monitor, then perhaps a UMPC is for you. To be honest, I think (and, indeed hope) that the next generation of machines will be cheaper (supposedly Samsung is working on a UMPC based on a cheaper AMD chipset, which will cost about £500) and have better battery life – and then I think that they will be a good improvement over something like a Jobo, offering more functionality for only a small increase in price. However, at the moment, I think the UMPC’s are either too expensive (the Sony) or too much of a bodge job (the Samsung) to be worth considering – although, it should be said, that if you are a pro, I imagine especially in something like photojournalism, being able to use a machine like a UMPC for tethered shooting could be advantageous for your job (interestingly several premiership football sides have wireless hotspots now, so UMPC’s could provide an elegant way to take pictures and upload them simultaneously, as the match goes on).