why does he take them on and off so much - is it a sign that he doesn't really need them but feels like he is clark kent (Gabriel) with them on and superman (Sylar) with them off?

yeah. they are a way of distiguisng his gentler side from the crazy brain stealing murder guy.

Maybe it is just a nervous habit, he seems like a pretty nervous guy.

Though I think if we were to back, we'd find that perhaps when he takes them off, he is having a Sylar moment or thought.

going off from what i said a second ago...the glasses are like a symbol, used alot of the time in literature and such.

He needs bifocals.

My guess is that the glasses are placed as a way to distinguish the more naive and not-so hungry Gabriel from the power-addicted Sylar.

They're brainyspecks, and Sylar is actually yet another incarnation of 5th/10th doctor!

He even wore chucks, at least in season 1.

It may be a way of showing when he is and isn't using his abilities.

Sylar has poor vision, by nature, but can correct them using his ability? In the latest future, Sylar uses them all the time until Peter asks him to pain the future.

We know that Adam's (and presumably Claire's) ability is autoheal, and that the autoheal keeps them from aging by renewing cells.. Sylar's vision could be deficient visual cells that his Claire-acquired ability can autoheal if he allows it to do so, but in doing so triggers his hunger.

Yes, I know that he even goes without his glasses before he acquires Claire's powers. I'm guessing he's using other powers (constant tk?) to fix his vision? I dunno.

That comment just made my day.

Like some of the people above said. Its a symbol, something that distinguishs his innocence (Gabriel) from his dark (Sylar). I don't think its used when he just uses his powers, its just when he does it's often at the same time he commits crimes.