I've never really used a rangefinder like a leica M series or the FEDs or Zorkis or anything similar. I've always either gone for the SLR or the dead simple point and shoot. I've always wondered what you see through the viewfinder in a rangefinder. It looks like you aren't looking through the lens. Is this true? How do you frame shots especially with different lenses?
The image you see will look more like what you'd get using a point-and-shoot than a reflex camera. Since you are not viewing through the taking lens, you will not see depth-of-field, exaggerated perspective, vignetting, etc.
In the finder image itself, you'll see framelines (sometimes more than 1 set) which approximate the field of view of whatever lens you have mounted. The M bodies auto select the framelines but you can override with the lever. There's a focusing patch in the middle. On rangefinder coupled cameras the framelines will move as you focus the lens in order to compensate for parallax.