Congrats,
I just bought the 551 about a week ago and I love it. I mounted it on a flat-top AR Bushy H BAR and it took hardly any time at all to make it zeroed and cowitnessed with my ARMS 40L rear BUIS, and the .040 new front sight post that Bushmaster suggests go along to match the ARMS 40 rear BUIS. I liked it so much that I will be receiving another one, this time the 552 for my other Bushy M4gery A2 next week.
When I was at the range last week a few old-timers asked if they could see what the sight reticle looked like. It was pretty cool to see their eyes light up and hear their comments about the reticle pattern. I thought it was well worth paying a few dollars more for the 551( N-type 1.5v batteries ) & 552 ( AA lithium, alkaline and rechargeable due to the night vision capability, and the other upgraded stuff. I can hardly believe that those two models also can be under up to 33 ft of water with no problem. Maybe that is some kind of SEAL requirement? Well anyway, congrats again, and I will be looking forward to using my brand new 551 and 552 also.