I found a lower that has a secondary set screw in the pistol grip for a trigger preset.  Has anyone used one of these and if so how well do they work.

Superior arms and the Socom Diamond lowers both use this. It simply applies pressure on the bottom rear of the trigger walking it out of engagment with the hammer recess. Thus shortening the throw of the trigger. It requires a little fine tuning but is a very nice addition for not furthur cost. Do a 10 min trigger job to polish the surfaces and it will be crisp and light.

It doesn't get you a perfect trigger, and like any other adjustable trigger, if you over-adjust it it can make the weapon unsafe due to insufficient sear engagement.

When wound way up, it can hold the tail of the trigger so high that you cannot remove the safety and possibly interfere with safety movement.

Don't get me wrong, it can give you a nicer trigger pull when the parts are polished properly and the screw is properly adjusted. I would not pay a whole lot extra for this feature in a lower, but it is nice to have sometimes.

Someday I'll have to compare it to the drop-in RRA two-stage.

http://www.kisermunitions.com/?page=shop/detail&product_id=215

If you look at the above it is a superior arms lower and it looks like a good price to me.  Just wanted to know about that screw.  

I guess I should have mentioned that my 2 SBRs are built on Superior lowers with that set screw in place and in use?

Be careful when adjusting it and check every so often to make sure the trigger & hammer are still engaging properly. It will make things nicer if you do your part.