hey chris, I just saw your question, sorry I missed it!
The 511 model is hated ( maybe that's too harsh, but if you keep reading, you'll see!) because it is the entry level model, no night vision compatible, and it uses N cell batteries. The big "problem" with it is the N cells...I have no trouble finding them and have built up a nice stash. They don't last very long compared to the AA models, those you can use the super batteries in and stretch their life out. I turn mine off when I am done shooting and they stay off until the next time, and there are no "supercell" N models to my knowledge...
I like them cause they are an inch shorter than the AA models and my first one on my first AR was a 511 and I'm sentimental. Plus, it's really all I need and I'm not into upgrading just for the sake of upgrading...rather spend my cash on more ammo and some training classes..