The PIX is definitely an enterprise solution. We use the FWSMs, 535s and 515 in our organization and they all perform well. The PIX is very reliable and is as good as any firewall.What is holding the PIX back is a true management solution. I know ...
I have seen the same thing in my environment. PDM was working fine then just stopped. I tried reinstalling PDM without any luck. I am running PIX 6.3(3) and PDM 3.01.
I typically use VTP password as part of the VTP domain configuration. If a device is plugged in and is not configured with the VTP password configured on the domain it cannot corrupt the database.