Crunch has pretty much covered it so I won't repeat. I have two 551's myself; one real and one from Huang. However, if you're looking for a really decent alternative to the EOTech style of sight, consider the Aimpoint Micro T-1 on a LRT riser. The real steel version, like EOTechs, is prohibitively expensive but there are pretty decent, airsoft-capable clones out there -- the MilSpex from RW is just one that comes to mind but there are others.
Food for thought anyway...
- Fides et Amicitia
"Can't keep my eyes from the circling skies,
Tongue-tied and twisted Just an earth-bound misfit, I"
Learning To Fly
© 1986 Pink Floyd