I've had a 24/7 set (with front and rear tritium) riding on my EDC Glock 17 for 3 years now. All day, every day. I've logged just under 7,000 rounds of shooting with them installed.
XS Big Dots are perhaps the most polarizing piece of shooting/self defense gear ever devised. There's a very controversial instructor/ training outfit in particular that's been pushing them for years. Andy Stanford of OPS, author of 'Surgical Speed Shooting' (pretty much the BIBLE of practical shooting fundamentals, IMO) endorses and uses them. On the flip side, there are a lot highly respected trainers that think they completely hamstring your potential for accurate hits at speed (Todd Green comes to mind, an absolutely phenomenal shooter).
I love them. I don't forsee myself carrying paper-shooting sights ever again.
These are gunfighting sights. They are designed to quickly and accurately index your handgun on a moving threat that's trying to kill you. They are incredibly fast to pick up, especially when you have the decreased short-range visual acuity that you get under stress. They are more than accurate enough to place hits at long ranges.
To me, there's almost a zen-like quality to using them. There's no examining a front sight post, checking its position in a notch, lining up the top edge, ensuring an equal amount of light on both sides of the post, etc...
With Big Dots, my mental monologue is simply a very fast mantra of "DOT, press, reset, DOT, press, reset". It's an unthinking, caveman state of being. It's almost like going on autopilot. You cover the target with the dot, and you'll send a hole through it, period (provided you have solid marksmanship fundamentals). While you're quickly breaking cover from behind a barricade, shooting from an awkward position, or hauling *** on the move while shooting, there's typically no need to stress about the position of the rear sight. Just use the dot like you would an Aimpoint on a long gun (though your focus is still on the dot, not on the target like one uses an Aimpoint).
All that said, I've seen a couple people that have absolutely abysmal performance using them. In my opinion, both had less than stellar markmanship fundamentals to begin with. The point is, they're not for everybody. Comparing them with traditional notch and post sights by shooting drills with a shot timer is the best way to give them an honest asessment.