This is the one I got. There is a clear one without markings as well.
Focusing Screen
Metering is very close to the stock screen. I use it with my f4 300. If you use slower lenses, it works, but don't buy it for that purpose. The stock screen darkens and gets blotchy in low light, this one just darkens.
It really shines with fast lenses. You can actually see whether it is in focus or not. I find it helps with autofocus lenses as well since I can see where the af mechanism is focussing to.
Focussingscreen.com sends a collection of shims. It isn't hard to change, a bit fussy and small, and it is worth the time to test it and get it right.