I was given a sonicwall Pro-VX that was decomissioned from a local bank
this week. I installed it at home and used it for a few days.
All I have to say is that m0n0 beats it to a pulp in almost every way!
I'm a fairly equipment/OS agnostic individual but the features and
function of m0n0 make it a far superior product. It also helps
that you can run it on pretty much any x86 hardware to account for
higher or lower performance requirements.
The only useful thing that the sonicwall will do that m0n0 doesn't
already is allow two firewalls to talk to each other via heartbeat and
one will takeover the interface ips of the other in case of hardware
failure to prevent downtime.