I have seen this post about the Raspberry Pi for use as an IDS. But how well would it do as a firewall appliance in a residential setting? I will soon be setting up a new network and want to exercise my knowledge of network security. I don't exactly have an old pc to dedicate as a security device. I would purchase a Pi 3 for this sole purpose. Any thoughts or experiences to guide me?
This all depends on how fast of an internet connection you want to run a firewall for. A firewall needs at least 2 interfaces, so you'd have to attach a 2nd USB ethernet device. Because the on-board ethernet on the Pi is attached to the USB bus as well, you'll have all the bandwidth over the one USB interface.
For a slow connection -- something like 20Mbit or slower -- a Raspberry Pi3 will probably work fine. If you need anything approaching 100Mbit or faster, it just won't happen over the interfaces available on this board.