In the realm of combat shooting the standard rule of thumb is, he who hits first wins. Consequentially, for the last couple of hundred years, the focus in firearms training has been sight alignment -- the faster you can align the front and rear sights on your target, the faster you can put lethal fire on your target.
Unfortunately, until recently, darkness presented a significant challenge to this theory. If its too dark to see your sights, you can pretty well guarantee that you arent going to be able to align them very well and your accuracy is going to suffer. While there are a number of ways to overcome this condition (illuminate the battlefield with flares, illuminate your sights with tritium or similar material, or illuminate your fire by using tracers) none has been universally effective.
Electro-optical reflex sights have changed all of that. These sights are battery powered, non-magnified, single sight optics that not only allow the shooter to rapidly acquire a good sight picture and alignment in all conditions (day or night) but also provide the shooter with better situational awareness since they need not be totally focused on aligning their sights, but rather need only put the dot on the target, literally.
The first of these sights was the Aimpoint M2 which the Army designated the M68 CCO (close combat optic.) It was driven by a watch-type battery and had a single on/off rheostat on the side to adjust reticle brightness.
While the sight did perform as advertised, I had issues with it. First, the on/off knob was easy to accidentally bump, which could either cause your sight to turn off or go to max power, which not only reduced your battery life, but also produced a visible red glow out of the back of the sight. Furthermore, the aiming dot was visible, at high power, through the front of the sight, which could reveal your location to an NVG equipped enemy (I am told this has been addressed though I cant confirm it.)
The sight I liked, and what in addition to the ACOG seems to be the one being currently issued, is the EOTech model 550. The 550 is a non-magnified EO sight which displays a 1MOA (minute of angle) dot in the center of a 65MOA circle. Reticle brightness is regulated by up/down buttons on the back of the sight, and the newer versions are equipped with a NVG direct button that automatically dims the reticle for use with night vision devices. The 550 is powered by a pair of standard AA batteries, has a reticle life of 1100 hours, and is waterproof to 1 ATM (33 feet).
What I liked about the 550 over the M68 was the battery choice (AAs are much easier to get) the fact that there is no forward projection of the beam, even on max power, and that there was more positive control of the reticle brightness. The 550 is also mil-std 1913 rail compatable and works well with weapon mounted NVGs such as the AN/PVS-10.
While Im sure that there are certainly more modern CCOs out there with many more bells and whistles, the 550 does everything I need it to and it didnt cost an arm and a leg.
(See much more "beyond standard issue" advice and tricks over at Kit Up!)
-- Eric Daniel