There are three basic models.
[url]http://www.rushusa.com/html/products/eotech.html[/url]
The lowest grade is the Bushnell Holosight. It lacks the "roll cage", which is a steel plate wrapped around the unit for extra protection in rough usage. It takes N batteries, which last about 70 hrs.
Next is the "tactical" series. These are have the protective roll cage. The 511 uses N batteries for 70 hrs operation. The 512 is the same, but uses AA Alkalines for 300 hrs, or AA Lithiums for 500 hrs.
The top two are the full "military" models, and have the roll cage AND are NV compatible. An extra button in the middle switches the reticle to infrared. It appears to go out, but is in IR mode, and can be viewed with NV goggles. The 551 is the short one that uses N batteries, and 552 is the longer one that uses AA's.
At the top by the Bushnell version you can see what the standard reticle looks like. The inner dot is 1 moa, and outter ring is 65 moa. This is quick to bracket a target, or zero the center dot on the bullseye. I personally like this better than the plain dot reticles offered as options.
I own one 552 and two 512's. I don't recommend stuff I don't use myself.