I have a Bushnell Holosight, Gen II. It uses N Cells. Battery life is very good, and I like the unit just as much as my Eotech that cost me double the money.
N Cells are 2.99 at Radio Shack for a two pack. 123's out here are 2.99 Each.
I put 3 or 4 pairs of N's into the pistol grip of my SIG 556... I can't remember and don't feel like opening the vault to dig it out and count them... but I can pack a lot of spare batteries on the gun. And they have no added heft to the weapon.
I think N cells rock.
The only downside the N cells is that Surefire doesn't make a flashlight with more brightness than a Solar Flare that uses them, so they are not considered "Tactical".
Well, there are tactical enough for me. They cost less, they are smaller and lighter so I can pack more of them for almost no added bulk or weight, and last long enough to take care of a crazy amount of business. My first pair of batteries lasted, what? All summer? Tactical enough.