How useful is volume in Forex?

Well VERY is the answer. You can see how how recent spikes in volume on the GBPJPY daily chart have preceeded some signifcant moves.

Remember the spike usually occurs on a bar opposite to the direction of the subsequent move - this is to be expected as large funds etc buy on downbars and sell on upbars.

If you use this for nothing else other than getting a general bias for direction, it can signifcantly stack the odds in your favour in certain markets especially when a short- or longer-term trend change is imminent.