I prefer the Apex Tactical rear sight blade over the factory rear sight blade -- it gives a cleaner rear sight picture. I find the factory rear sight to be busier than the Apex, and thus more likely to attract my eye (which should be focusing on the front sight instead).
The factory notch size is 0.125", as is Apex Tactical's. I had Apex Tactical open up the notch to 0.140 for a small fee.
I did not install it myself. I took one look at the tiny size of the screw, spring, and the other part (plunger?) and decided that I'd drop and lose them. So I paid Greg Derr to install it. Another alternative is to unscrew your entire rear sight from the gun, ship it to Apex, and have them install the new rear blade. From what I understand, installing the rear sight is not hard, but man those parts are small and I just don't have the patience.
Apex Tactical doesn't have a picture of their rear sight on their web site, but it looks a lot like the Weigand rear sight:
The main difference between the two is that the Weigand blade is aluminum and thus not as sturdy as the Apex blade, which is made of steel. You can order the sight blade from Apex (call them up). The sight blade comes with the required parts from Apex.
Apex Tactical is here:
http://www.apextactical.com/
