The XS Express Sights are danged quick to
see, but require a different sight-picture for different ranges. At close range, you just cover your intended impact area with the big dot, but at longer range (25 yards, for example) you have to use a 6 O'Clock hold to hit. This works fine, after a bit of experimentation - but requires a bit of decision-making while already busy shooting.
Like M.B., I was exposed to them at Gunsite (where they had some very clear posters illustrating the different sight-pictures required at different ranges). Since my eyes don't seem to be teenagers anymore and are taking longer and longer to snap a sight-picture into focus, I talked to the folks in the Gunsmithy about installing a set of XS sights. They asked how long I'd been using regular sights on my pistols ('bout 30 years), and suggested that it was a bit late in the game for me to start trying to learn a new sight-picture with a built-in zone-management requirement.
What they suggested was that I try a compromise they call their "Old-Guy Sights" - an XS Big-Dot with a correspondingly sized round notch in the rear sight, which permits a conventional sight-picture with the top of the front dot level with the top of the rear notch. There was a comfortable bit of daylight showing around the front dot inside the rear notch, and it didn't look as if it could possibly be very precise. Corey let me try a couple of magazines through a shop-gun with this set-up on the 6" plates set up 25 yards behind the Gunsmithy. 15 satisfying "Clangs" later, and I was sold on the idea. The equivalent of "head-shots" as quick as I could spot'em. Just cover the gong and press the trigger - I was shooting like I was 20 years younger!
So, if you're not too set in your ways, the factory XS Express Sights are definitely a good thing. If you ARE a bit set in your ways (I prefer to think of it as a set of carefully cultivated conditioned-reflexes

), you might consider the Gunsite Old-Guy Sights.
-KBS