I've got a Kobra on my M4gery and my SAR 2, and love them. They're sturdy, reliable, and the dot is small enough to actually allow reasonably precise shooting, should I need/want to do it.
I've never used the OKOs, but people speak highly of them. If you have a C&R or FFL, Brownell's has 'em for $150, and another $53 for the cover. One nice thing about the OKO is that they have a primitive back up sight built in...I don't know how functional it is, but at least it's there.
If you opt for the Kobra, get the quick detach mount..that's the only decision I regret, I got the other mount...it's not that bad, but not as quick as it could be. Tantal has the Kobras for around $130, mount included. He is a standup guy and worthy of your business.
Some folks think the Kobra sits up too high, but it's not that bad. Works for me. Here's a pic:
[img]http://img.villagephotos.com/p/2003-5/188491/M4_1.jpg[/img]