Yes, Paul's correct, magnification is a problem. It is with the prisms, too, as they are just 3x. In fact, the WLF is best in this respect, at 4x magnification. This has only really become an issue in recent years, with the advent of digital backs and their forensic treatment of focus.
But it all depends on your intended use, as well. I always use the chimney finder on a tripod for still-life and close up work, and because I use the longer lenses for this (120 and 150mm) they are a lot easier to focus with, especially close in. You certainly would not want to use the chimney finder with the Distagon wide-angles for landscape or architecture, where spot focus was critical.
The best screens are also essential, of course. For digital you must use the latest Acute Matte screens, and some folks find the ones with the split prism helpful.
John