I have a Cisco PIX of some model or other, I forget which, think it's about 3 or 4 rack units tall to which I added a hard drive, a new MB, a couple gig of RAM, a P4, a couple gig NICs (and a fiber gig NIC as well), a new PS that supported the extra power connectors the P4 needs, and an extra fan or two. So it looked like a PIX until you took off the case or connected to the console and saw it was pfSense.
I never actually put it into use, as I wanted to button it up a bit neater inside, but just never got around to it. Mostly did it just to do it. It makes for a fairly nice pfSense box, but I just bought a WatchGuard XTM5 (running pfSense of course) that I'll be putting into place when we move to the new house just because it's a bit more powerful and looks a lot prettier. And it only takes up 1u which is nice when you're using a small wall rack.