Well, the camera I started building on Friday is now complete. The build wasn't that tricky, although I did end up making the lens assembly four times until I got it right.
The viewfinder is kind of interesting. The view through the lens is projected onto a little screen, just like a "proper" single lens reflex camera. However, unlike a normal SLR, the viewfinder image is not further reflected inside a pentaprism. What you see on the screen is the right way up, but flipped left to right, which took me a while to figure out. The image is also a bit dim, so you need to assemble a little screen around it to keep the light out.
But it works. You can see to focus and compose your picture and I've loaded the camera up with film and taken a few shots. Everything is delightfully primitive and I'm looking forward to taking the film in and having the pictures processed.