I mounted an XS-Express setup on my PX4 .40; it has a more normally sized front sight, as opposed to the XS Big Dot, but still has the shallow rear trough. The rear sight also has a tritium bar in the center, allowing quick front-rear alignment in low-light or no-light conditions. (Caveat: No-light doesn't allow good target identification.)
Pros to the setup: Fast target acquisition; potentially accurate to mid-range.
Cons: In the case of my pistol, the dot sits on POI, not just below it, so small bullseyes and small targets are obscured or hidden by the front sight; wide rear trough with center bar work well enough for lateral alignment, but aren't as helpful for vertical alignment at more distant ranges.
So, based solely on my experience with the PX4, I'd say the XS setup is good for close range, fast acquisition shooting; I'd recommend it for smaller guns, that many shooters don't plan to use at longer ranges - put the dot on the target and pull the trigger.
But for a more well-rounded setup, I'd go with Heinie Straight-8's, or Heinie 10-8 with front bead. The rear apertures of both of those are better for finer aim over distance.
Or else I'd go with a regular 3-dot trijicon or meprolite setup.
YMMV.