This is a photo of the completed, and operational, CNC machine that I've been building for far longer than I care to admit to. It is the Black Toe model (24" x 48" x ~5") from Build Your CNC and is connected to an off-screen computer via the grey cables. It uses an off-the-shelf Porter Cable router and can use any router bit that will fit in the collets (up to 0.5" shank and 10 000 to 23 000 RPM). Structural components are made from MDO and drive rails are aluminum.

I am led to believe that this machine can be calibrated and squared to be repeatable to 0.1 mm. This is great as I intend to mill PCBs with it to avoid the use of dangerous chemical solutions.

Motion is achieved through three stepper motors. The X and Y axes are roller chain driven and the Z-axis is driven by lead screw. Speeds over 1000 inches per minute are possible but not recommended.

I have used it to rout text and to cut out shapes and make grooves in MDF, pine, maple, and maple plywood without much issue. I've yet to try oak or plastics and it is also supposed to handle hard foams (or anything that the router bit can get its teeth into). Metals that can be routed are aluminum and copper but steel will likely cause problems. Glass is right out due to the liquid cooling required and the use of MDO in its structure.

I'm still learning how to make more and more complicated shapes using my CAD software and it likely needs to be recalibrated now that it's been used for a bit and all the motion components have been worn in but it's very powerful and I can't wait to start making nice things for people.

A few comments:
It is loud! When the router *AND* ShopVac are both running, you can't hear yourself think.
It is delicate. Bits can break easily if you do not take care with them.
It is powerful. Not a contradiction with the previous statement. The router is rated to 12 Amperes (1440 Watts) and it will just as happily push the bit through a sheet of plywood as it will through your finger. Respect the power.
It has no brain of its own. I have to use mine to think about how I want it to move and then to program it accordingly. I have to find a good way to install soft and hard end limits to prevent crashing the gantry into the end stops. Until then, I need to be its eyes and ears.
I've named it Applejack. Yes, *that* Applejack.

If anyone has experience with these sorts of machines, home-built, kit-built (like mine), or professional, feel free to leave a comment.