strongmad, if you're talking about a Kobra, they're great sights for what they were designed for...the replace the crappy AK iron sights. I've had one on an AR, and it's shoots great...but they're big and bulky, and with the hump on the back, you really have to look over the top to get a good sight picture. Plus, there's no way to use a BUIS without removing the whole thing.
I just got a Holosight for my M4gery, and I really like it. It has the best or least obtrusive sight picture of any of the sights I've tried.
Now, the Eotechs are the LE/Military version of the Holosight...I'm sure there are those here who can tell you what the difference is...and the Eotechs well be more expensive. You can get at least one of them that takes AA batteries, but the one I have takes the small 3v ones. (dont' remember exactly which ones).
Holosight or Eotech, I think they're the best bet for what you seem to be looking for.