I have recently purchased the Window cleaning warehouse backpack, had been meaning to for some time...
I have a full van mount by the way.
I have found it absolutely first rate, I can get 2 semi's done with it, that is with it filled to the brim, officially it holds 15l but I guess when brimmed it is closer to 16l.
Oh, and I always wash the frames and doors too by the way.
On drive between cleans fantastic, on large jobs then use the van mount or a combination of the two, depending on how far you would have to drag out the hose.
As an extra tool in the van it is fantastic, as a stand alone piece of equipment I'd not be so sure, it would rather depend on the nature of your work and the size of your accounts.
I started out with a trolley system over 4 years ago, it soon mutated into a mini van mount system in the back of my car.
I'd say that a backpack is a better way to go than a trolley system.
Some of the trolley systems can weigh a total of 7 stone or more when the weight of the trolley, the water, the battery and so on are all taken into consideration, that is a lot to lug in and out of your vehicle all the time.
Having said that, I'm now a very experienced WFPer and use minimal water, so I can make the water in the backpack go a long way.
I'm fitting a transfer pump into the van mount tank, this should mean I can refill the backpack when it is used in about 15 seconds, and if you have dragged out 30 metres of hose it can take you at least that long to wind it in, plus of course when the backpack is full, I just open the door of the van, grab it and the pole and walk straight to the first window.
I know you also do this with the hose, but I don't have to think about obstacles and plan my route with the hose, so in that respect also the backpack is quicker.
I now think I have the ideal combination, van mount and backpack, I've got just about all the angles covered now
