About 6 months ago you recommended that I get a EOTech 551. I was at the Houston gun show last week, & I finally got a chance to look at 1. The guy selling them recommend the 511 instead, because it was $100 less, & does the same thing except it's not night vision compatible, but I don't have night vision. So would you still recommend the 551, or would the 511 be just as good for me? Thanks SMG Lee.
Searcher out

I'm not Mr. Lee, but I think I can help. The 550 series has the rubberized buttons instead of the membran touch pad controls.  The are better for use with gloves or in darkened situations.  

Also, if you do sub lock outs, water pressure won't cause the settings to change which was a problem with the membrane type interface.[0j]

Can you hook me up with a sub, so's I can test it out for my self?[:D]

The difference between the 551 and the 511 is pretty mininum for a civilian shooter.  the 511 is the LE sight and the 551 is the military sight.  the 551 have a dive rating of 33ft for 24 hours and 66ft for an undisclosed time frame.  The rest is pretty correctly covered by Knightone.

If you are just looking to shoot for fun, the 511 will be more then sufficent. if you do plan for operation in extreme cold weather or night time operations(hunting) you might look into the 551.