Part of the reason that ID is such a seductive argument is the way that adaptation is often referred to in popular culture. No natural history documentary script would be complete without a description of the polar bear as "perfectly adapted" to its habitat or a eulogy to the "exquisite camouflage" of the arctic fox.
But nature is much more interesting than the Discovery Channel would have us believe. Look closely and much of it wouldn't be winning any design awards. Rabbits, for example, have an inefficient and frankly gross way of digesting their food. Your furry pet has a side branch to its gut that is full of enzymes and bacteria. By munching on half-digested morsels from this side branch that have passed out of its backside the rabbit's stomach and intestines have a second go at extracting nutrients. It works, but from a design point of view it is crazy.
Despite looking pretty impressive, the human eye itself is put together in a way that a fairly unintelligent designer could improve on. The rod and cone cells that gather light and convert it into electrical impulses destined for the brain are wired up "back to front". So light hitting the retina has to pass through a maze of wiring before it reaches the light-gathering rods and cones. And anyone with a bad back could be forgiven for cursing our knuckle-dragging ancestors who gave us a spine that is not well designed for upright support. The point is, though, that all of these bad designs, with their echoes of ancestors long since gone, are good enough.
And as to the old creationist argument that half an eye (or half of any complex structure) would do no good, well, consider that one BUSTED. A rudimentary light-sensing patch with 4x4 resolution is far better than no light-sensing equipment whatsoever; a cochlear implant with 16 channels is better than no hearing at all. It's easy to see how just a single cell with the ability to sense where the light is conferred a huge advantage on the organism that posessed it. So too with the first animal to evolve a two-celled light sensing organ. Two becomes 4, 4 becomes 8. Add some protective extracellular material, wire up some neural networks, and add a couple hundred million years of hardcore selection for the ability to sense one's environment more and more acutely... poof! the vertebrate eye!
"Nothing in biology makes sense except in light of evolution." T. Dobzhansky