History and Evolution:
Red Dot equipped pistols have been around for over two decades. In the 1990 USPSA Nationals Jerry Barnhart used a red dot scope to win and the same year Doug Koenig won the IPSC World Shoot with a red dot equipped CZ.
The first defensive gun I saw with a red dot was in a Kelly Mccann video back in the late 90's. He had a Doctor Optic mounted to a Glock 19 and I thought it was the most revolutionary thing I had ever seen. After getting one and shooting with it, I quickly lost interest. The window was too small, the optic was too fragile and I had no back up sights.
Fast forward to 2010 and I was reading about Gabe Suarez mounting an Aimpoint T-1 to a Glock. While I liked the idea, the T-1 was still way to big for a carry gun.
A few years earlier, David Bowie of Bowie Tactical, was starting to mill Glocks and M&Ps to accept micro red dots. The major difference between what Bowie and Suarez offers is where the rear sight is positioned. David strongly prefers to put the rear sight in front of the optic and Suarez prefers to put it behind the optic.
In the last year an exciting new mounting option has come out from Unity Tactical. The ATOM mount allows users to change the mounting plate to accommodate the use of different optics. I see this as a critical feature moving forward to provide a platform that can evolve with optics technology.
Presently the best option (in my opinion) for slide mounted red dots comes from Trijicon in the form of their RMR (Ruggedized Miniature Reflex). The RMR provides the end user with an extremely durable, small, lightweight design that works well with most pistols. Other options such as the Leupold DeltaPoint are OK but the general consensus is the the RMR is tougher and more user friendly.
The RMR comes in 3 main configurations:
The Dual Illuminated model I have uses a fiber-optic collector as well as tritium to power the illumination. This model work in about 99% of shooting conditions, but can be a problem when shooting from a dark area into bright sunlight; one reason why back-up iron sights are so important! The Dual Illuminated model comes in Amber and Green dot configurations as well as a triangle reticle. The reticles vary in size from 7-13 MOA.
The LED version, recommended as the best version by people "in the know", is battery powered and automatically adjusts to light conditions. Trijicon claims the battery lasts for 2 years under normal conditions. Early versions had issues with the wiring shorting out, but Trijicon has rectified the situation since. I highly recommend buying the RM63 seal from Trijicon, it keeps water out of the battery area for up to one hour while submerged to 66ft. or two atmospheres. The dots on the LED models are red and come in 3.25 or 6.5 MOA.
The adjustable LED model has been criticized because it has a tendency to get inadvertently turned off by holsters and other incidental contact. other than the ability to manually adjust the LED brightness, it is the same as the other LED model.
At the range:
Let me first say, there is a learning curve to shooting a pistol with a red dot optic. Don't expect to pick one up and be shooting better right away.
Upon shooting, the first thing you will probably notice it how much the dot feels like it is moving around; this is the same thing most people notice when first using a laser. Don't worry, your iron sights are moving just as much, you simply don't notice it.
I have spent about a year working with the new RMR/Glock and more recently an RMR/M&P they both have pros and cons. The biggest drawback is speed at close range; I have spent 20 years perfecting the ability to see notch and post sights quickly, at close range and transitioning to a red dot has proven challenging. That said, my hits are more accurate on average and I can make precision hits much better at longer ranges.
While I haven't had any issues with the function of the gun, I have had issues with the RMR05 holding zero. On a number of occasions, after firing between 100 and 500 rounds, the windage and/or elevation adjustments have drifted. I have sent it back to Trijicon 2 times for repair and they supposedly have new turrets installed in more recent production models. Trijicon service has been great and I have confidence they will fix or repair the RMR. My RMR02 has been lawless.
From a weapons manipulation standpoint, having the RMR slide mounted doesn't change much, but it does make one handed reloading and malfunction clearances much easier.
There are some great write ups from Hilton Yam at Modern Service Weapons on the Pros and Cons of red dot equipped service pistols you can also see a number of great threads on Pistol-Training.com.
Here is my Pro/Con list for the RMR
Easy to use with worsening eyesight
Good visibility in all lighting conditions
Increased accuracy at range
Improved single hand manipulations
Reliability (seems to be fixed on newer models)
All in all, I think red dot equipped pistols are the future of defensive handguns. The teething problems we are seeing with sights are the same we saw with early red dots on rifles. Time and testing has shown that red dot optics on rifles provide end users with better ability to, hit the target in all lighting conditions, shoot more accurately and be rugged enough for combat use.