I wish I could reply to this with more scientific backing, or more emperical data, but it's all observation on my part...
My cobras albino monocle(Naja Kaouthia) and Pakistan (N.N.N.) do not appear to see very well. In fact, the monocle acts as if he's blind sometimes, but then he'll surprise me with evidence of vision. The Pakistan is not as bad, but I wonder about her eye sight at times...I say that to offer up an excuse as to why they may not be as attentive as the mambas...
I have a D. Angusticeps (Eastern Green) and two D. viridis (western Green). The Eastern appears very intelligent as her eye ball follows my movements and approaches me each and every time I come near her. She is very curious--flighty--but curious. She explores the areas I may have just entered and adjusted or cleaned. She is very aware of my movements and can remember where I have been--it "seems."
The westerns are just as curious. They also approach the glass as I near it, flicking their tongues of course, but still remaining very visual. They don't appear threatened, just interested in what I'm doing.
I know none of this proves anything and it's all just subjective observation on my part, but the impression I get is that they are more "intelligent" than the vipers and perhaps other lizards.
The cobras are different. They don't seem to asses danger as well and will become defensive quicker. But if they relax, they are also "playful," but not in the entertaining sense, but rather in the curious sense, like cat-and-mouse games. Again, this is all relative. I find it very interesting to watch the Kaouthia eat. He'll strike the rat once--and only once. Then he'll back off and wait for it to die. I "knows" that it will die and knows that there is a fatal consequence to its bite and that it takes time to kill its prey. I've observed the rat (numerous times) in a corner, hiding after it's been bitten. The cobra slowly goes up to it, sees it's not dead, and backs off for a while. He'll do this until the rat is dead. It never strikes again.
The Pakistan, however, will strike repeatedly and violently. She wants her prey to die and die "now." Both western mambas strike with repeated strikes, but considering how fast the venom reacts, the rat has only 30 seconds at best anyway. The eastern seems more patient, striking and willing to track it down later.
Anyway, those are my quick observations.