We've got a couple of them that we use for granular material - fert, snapshot. I was wondering how they would work with liquids so I grabbed one the other day and tried it out with some water. Setup is easy enough. The spray pattern is ok, but if you ask me, the biggest downfall is that the tank is pretty small and mounted at the top of the unit. When walking around, you really notice the liquid sloshing around near shoulder height. I also feel like there is less control than when using a gun where you can adjust the pattern.
I currently use a Solo 433 gas powered backpack with a JD-9 gun attached. I absolutely love it. I originally got it for deer sprays, but it works so great that we use it for any of our smaller applications - roses, boxwood leafminer, dogwood/crabapple fungicide, etc. If needed, I can reach about 30 ft. height, although if you're really trying to get good coverage on something that big, volume might be an issue. It is often easier to put 5 gallons in the backpack than to snake the hose around everything in the back yard.
It's got adjustable pressure, a 1 hp honda 4-stroke motor, and a 5+ gallon tank. Add the JD-9 gun and the Rainbow injector and it is absolutely top notch. Not cheap, but it'll pay for itself in no time. http://solousa.com/store/flypage/motorized_backpack_sprayer/433_.html