I have used NetScreen firewalls in the past (waaay past, and recently). I've been pretty disappointed by them.
Historically, NetScreen used the wrong gender for their serial console ports. It's a little thing that shows a lack of understanding the problem space. (Juniper fixed this problem)
NetScreen's CLI is *awful*. It's very clearly not intended to be the way you really configure the box. It's very clearly not built by CLI people. They really want you to use their web UI. It's an okay web UI, and all, but fundamentally, real network gear is configured through the CLI and toy network gear is configured through a web UI. They've made a choice there.
Every month or so, I'd see a NetScreen in the field reboot for no apparent reason. Which one did it would vary. While not the end of the world, this too is not a good sign.
Rules are straightforward enough, VPNs work well enough, performance is reasonable for the price.
I just always felt like NetScreen didn't get it.
(BTW, Juniper firewalls / SSGs = NetScreen. Being owned by Juniper has seemed to affect them positively in terms of design clues and reliability)