If you have the hardware to build a firewall on (ie, a very cheap old PC with two network ports or cards + a small consumer grade network switch) and the expertise to do it, then you should definitely go with a homebuilt firewall instead. You get great stability and upgradability along with far better features than routers usually offer. Another thing you get that is either good or bad depending on your viewpoint is a split of your network hardware, ie instead of one router with everything (switch, router, wireless lan) you have separate parts for each. I prefer that since I can either upgrade one part if I want or replace any one part should it fail.
I've used Smoothwall and it is an excellent product, but I eventually switched over to
m0n0wall because it offered the features I needed and can be run entirely harddriveless. You insert the boot CD and a clean formatted floppydisk; it boots from CD and writes settings to the floppy. The entire firewall is held in internal memory so disk access is extremely infrequent and management is simple via a nice web interface.
One thing it offers off the bat is traffic shaping. This is great if you have to share your Internet link, as it prioritizes some stuff; this will keep the Internet connection from getting "clogged" even if it is being used 100%. Also, unlike on most routers, this is a real firewall and can block traffic both ways so you can set up rules so that the machines inside the m0n0wall can only, say, websurf and check mail via the POP3 port... stuff like that. Optionally, that is.
Obviously there is more, like VPN:s, NAT, etc etc; all in all a fantastic product and it's free.
