A homebrew Linux router is right for a hobbyist or junior sysadmin who is genuinely interested in how these things work under the hood, though. Setting up and managing one will teach you a lot. It's also a pretty good fit for a veteran sysadmin who already understands the majority of the systems and only has to brush up on a thing or two to get comfortable with it—the raw performance is just plain jaw-dropping, and the simple nature of the system will leave that veteran sysadmin free to manage it and back it up in the ways they're already very comfortable with. And finally, it's perfect for the incredibly tin-foil-hatted types (also, unfortunately, like me these days) since its stripped-down nature lets them be absolutely certain they understand what it is, and isn't, doing and plan around that accordingly when they're mapping out their security setup.
The Ars guide to building a Linux router from scratch: