Using linux firewalls for PCI compliant infrastructure

Hi

We are using linux-based servers as firewalls for PCI compliant
infrastructure. During audits it has been OK so far but security
people internally have suggested that maybe a commercial product would
be better suited for PCI infrastructure (as it is pretty critical).

I'm personally very happy with the iptables firewalls - we can use all
the standard components for firewalls that we use for everything else
(including standard administration methods, patching and so forth).

What do you think, would a commercial firewall provide a tangible
improvement in security?
Is anyone else using linux-based firewalls for PCI (or otherwise
sensitive) infrastructure?

Thanks,
Siim