We have a Stihl 450, and it does very well with anything I've put through it, including surround, sulfur, neem oil, compost tea and synthetic fungicides. It will happily spray many things our 3pt airblast won't, and has (thus far) proven to be a great and reliable machine.
The downside is that it has a very high center of gravity, with the tank at shoulder level, and is quite heavy. Walking while wearing it takes some getting used to.