While it's true that sometimes Barack Obama takes long pauses to grasp for a word, I don't think it could be argued that he is incoherent without assistance.
If that was the plain truth, it would be more obvious to the public by now. Especially after over 20 debates, and many being one on one with Sen. Clinton. The fact is, he's been in plenty of unscripted formats where he has performed well and exercised mental clarity.
If anything, I've observed that he is extremely tentative as not wanting to say the wrong thing.
Anybody who doesn't think Obama has proven at least something in defeating the Clintons is writing off a very a big deal.
Obama may steamroll over McCain, and if he does, he'll have become President by largely having easy electoral victories in his rise to the top. But he'll still have to his credit all of the planning and tactics that defeating an inevitable candidate took.
And that was probably the biggest part of this whole process for him. Getting the Democratic nomination in 2008, with few mistakes along the way, is a near certain assurance of winning the White House.
He's run a great campaign to this point, and it is a reflection of his qualifications and the ingenuity of the people he works with, even if he's untested in many other ways.