Icke is actually quite intelligent and well-spoken. But the problem is that he has become a true believer in shape-shifting reptiles and alien origins of humans, without any concrete evidence to support it. His belief is based primarily on things such as:
1.The personal accounts of various people he has spoken with over the last few years.
2. Reptiles depicted in art, architecture, etc.
3. "Gods" which interbred with primitive humans, mentioned either directly, or as Icke may interpret through anecdotes from ancient, historical texts (Sumerian tablets, The Bible, etc.).
4. The tradition of Royal bloodlines: these are reptilians who maintain the proper balance between reptilian and human DNA through cycles of inbreeding and "out"breeding in order to preserve the "shapeshifting" through generations of the family line.
These points are simply not solid evidence for the existence of reptilians. He shouldn't think it is enough to warrant such a strong belief in reptilians, when compared to the more solid evidence he has required in the past to support his arguments for other events and issues.
But, I don't think he is a "nut" or a "kook" solely by virtue of his belief in reptilians. History shows that many people were ridiculed and called lunatics for extreme ideas but some were later proven to be right - Galileo, Pasteur, the Wright Brothers, and so many others - both well known and obscure people. I'm not implying that Icke will one day be proven right, or that his name will be spoken with reverence in 100 years time or some such thing. I admit, if he
were ever proven to be right, it would surely have to rank as more revolutionary, incredible and astounding in scale than anything in recorded history. But personally, I don't think his theory is valid, and the odds of it one day being proven true are probably slim to none.
"Don't sail too far in one direction, you crazy fool - you'll fall off the edge of the Earth!"

This segment from a link posted below has some good points...
Although the crazy ideas usually prove to be just that, every so often they do prove to be correct. They even occasionally prove to have immense value. They trigger scientific revolutions. Therefore "crazy" ideas must never be automatically dismissed out of hand without first inspecting their supporting evidence. If we ridicule crazy ideas without giving them a fair hearing first, sooner or later we'll ridicule the next Galileo.
It's a bad practice to use sneering and ridicule to block unconventional ideas. It's a very bad idea to erect near-insurmountable barriers against all seemingly irrational ideas, because doing so will discard the occasional Galileos and Arrheniuses along with the large hoards of crackpots. Or in other words, never ridicule things which you've haven't even bothered to investigate.
Some Skeptics would prefer that the apparent crackpots always prove to be just that: crackpots. But the reality is not so simple. If we fight too hard to eliminate the "weird" stuff, then we run the risk of suppressing the next Copernicus. I say it this way: There are diamonds hidden in the sewage. Those who make it their job to keep Science entirely and totally "clean" will also become the newest generation of scoffing debunkers who fight to prevent the next group of genuine, disruptive, revolutionary advances.
The history of new ideas proves this by example. Any inspection of science history will reveal a long list of genuine discoveries which were treated with hostile prejudice. If skeptical people ignore such powerful evidence presented by "the crackpots," and if they instead distort the historical examples and proclaim them to be a simple logical error, then it is the skeptics who make the logical error, not the crackpots. When a crackpot drags out the old "They Laughed at the Wright Brothers" argument, that person is not stating that ongoing ridicule proves crackpot ideas must be correct. Instead that person is simply saying this:
"You who make a policy of automatically rejecting 'crazy' ideas without first giving them a fair hearing, you would have joined the experts in 1905 who refused to view the Wright Flyer in action, and whose continuing public ridicule eventually forced the Wrights to abandon the USA and move to France."http://www.amasci.com/freenrg/arrhenus.htmlIf nothing else of value, at least Icke and his lizard theory makes for interesting discussions!