Originally posted by tripodquest the pentax stock screen is really dark, there's no denying that,
The Katz Eye is no brighter even with the Optibrite treatment. The problem with the stock screen in not brightness, it is apparent DOF. You can demonstrate with this with your 50/1.7 using the optical DOF preview. Set the aperture ring at f/3.5 and actuate the DOF preview. Do you see a difference? Unless things have changed with the more recent bodies, you probably won't. In fact, you may not notice any appreciable dimming of the viewfinder. Now consider the difference in DOF between f/1.7 and f/3.5 and realize that the image you are using to focus with at f/1.7 has an effective DOF of f/3.5 while the image on the sensor has DOF for f/1.7.
Katz Eye makes a good product, but I would also consider focusingscreen.com or cut down your own from available Nikon or Canon options. I would cut my own before using any of the eBay vendors.
As for perspective on fast lenses, my signature is not real complete. I don't own a f/1.2 lens. I consider them a bit of a novelty item and poor value for general shooting (heavy, bulky, expensive, etc.). They are nice for abstract floral work and soft-touch portraits though. I do own a Rokkor (Minolta) 58/1.4 and Auto-Rikenon (Ricoh/Tomioka) 55/1.4. Both are nice lenses with the Rokkor being the better of the two. I used to own a Super Takumar 50/1.4, but found the performance disappointing for the scant 1/2 stop jump from my ST 55/1.8.
That last point is the important one. Almost all f/1.7 and f/1.8 Pentax (and Asahi) normal lenses are optically better at all shared apertures than their f/1.4 brethren. The f/1.2 and f/1.4 lenses generally are pretty soft and lacking in contrast at maximum aperture. My f/1.7, f/1.8, and f/2 lenses are ALL better than my two f/1.4.
Steve