A red dot sight makes a huge improvement on an AK. I consider it nearly a must for anyone serious about the platform. I opted for the PK-AS over the Kobra because a) it can be used without battery power and b) it looked a little more versitile in that the reticle allows for a good deal of precision in addition to being very fast. The pluses are that it is quick and easy to install, fast, and accurate. It seems to be very durable. The 1 MOA dot takes up very little of the target. I have shot it out to 300 yards and the small dot allows for easy hits at that range. The large circle is 44 mils and can be used as a rangefinder. It also is very quick and instinctive at close range, partially because it is used both eyes open.
The minuses are that the intial adjustments are akward, making the sight a PITA to sight in. This is especially true for the elevation adjustments since the screw that makes such adjustments in located on the bottom of the mount. Either the sight or the receiver cover must be removed to make adjustments as the included wrench can't reach the screw with the optic attached and the receiver cover in place. Other than that, the sight does sit too high to allow a solid cheek weld--at least the with stock I have in place. This is relatively minor since the both-eyes open style shooting this sight favors doesn't really require a cheek weld, but I will admit that it is still somewhat of an inconvenience. I should mention here that my PK is the "V" model that mounts directly over the bore, instead of slightly off to the left like the standard sight.
The Kobra appears to sit lower and might be slightly faster in CQB scenerios due to its tubeless, open HUD style construction. I haven't used it but from the reports I have heard it isn't quite as good at medium ranges as the PK-AS.
Regardless of which sight you choose, I should warn you. Stock up on ammo. If you haven't used a red dot before, nothing will prepare you for how much fun they are and being able to achieve such quick, accurate shots with the AK will make ammo seemingly evaporate.
Here's a pic of the view through the PK, taken from across my bedroom about ten feet from target: