I was on the fence about getting XS Big Dots, then heard some negative feedback. Most of this feedback was "the big dot won't be good for shooting at distance". After hearing that, I cooled on them a bit.
Then, I had a chance to run them FAST, in defensive scenarios, Hogan's Alley stuff, slow fire courses of fire, and 3 second par courses. I was very pleasantly surprised with them. They are easy to track, and I got to observe something funny that gets overlooked:
The Big Dot is huge, but the dug out rear blade actually opens up the total sight picture, so you can center the front sight *easier* than you can center the sights with a typical rear blade.
Look closely at this pic. You'll note how easy it is to see the target around the dot, with that rear blade out of the way.
Needless to say, I now have them on my carry Glock 19 Gen 4, and may get them on my second copy, as well.