I have one of these scopes on my AR15 flattop. You will definitely need an extended base due to the eye relief. It is a great scope and I recommend it highly. I bought mine from Lock, Stock, and Barrel at http://www.lockstock.com
They have some of the best prices on Leupold and have been in business many years. I have bought a lot of stuff from them. I think I paid around $620 for it a few months back. This is the version with side focus.
You have a variety of reticles available in this scope. I bought the standard duplex and it is ok. If I were to do it over again I would look at the target dot version - a little more expensive. I have been thinking about shipping it back to Leupold to get an alternative reticle installed.