Charlie,
The Firewall Feature set with CBAC is an excellent choice if you want a one box solution and least cost of training, but remember that it is a router first. The PIX on the other hand is a Firewall first and doesn't have the overhead of routing, ect. Becasue of this, you can be more flexible in the implementation of your Security Policy with the PIX. In most of the implementations I have done with the Firewall Feature Set, it was to supplement the PIX, not replace it.
