NetScout is unique in the world of Packet Flow appliances and related technology.
While not a widely-known name, they have been in the packet analysis business for over 20 years. A few years ago they acquired Network General, the company behind the Sniffer brand, and became the super-vendor in this market space.
Many APM vendors have a broad range of products that try to address all of the different categories of APM functions, of which packet flow is just one. They usually have one or two strong functions and a bunch of “me too” functions that they got from an acquisition a few years ago, and which are not integrated properly.