Niksun Probe

There is also the INVEA-TECH Flowmon :

That's a radically different thing. Niksun, NetWitness, Solera are all about capturing lots of packets at very high speed. INVEA-TECH is a NetFlow kind of thing. Totally different; tells you completely different things about your network.

If you want flows, there's LOTS of great products for flow monitoring, starting with Cisco and moving your way out through both open source and commercial products. But if you are considering Niksun, then flows are not what you're after. Packets are what you're after.