I know his problems in the past, but overall he's an OK CB who rarely gets beat clean. Plus he's super return man.
McGee is shot, pretty much out of the picture from the Bills viewpoint. From my eyes, Gilmore is getting better each week, and has the look of a shutdown corner (not flashy but solid). Brooks is unknown and will remain unknown for awhile. Rogers proved to be 4th CB material at best so far. And I know they are going to draft one next year.
Aaron Williams may have everything you look for a CB except elite speed, which seems to get into his head too much now. Instead of focusing on jamming the receivers, knocking them off the route (his strength), he's tentative, afraid of being beat, which leads to more trouble. I think he might be, just like some draft guru predicted, best suited for safety. I know the Steelers were looking at him during the draft, and some compared him Ryan Clark. At the safety position, his speed becomes a strength and he can play more aggresive, maximizing his strength.
In that scenario, having McKelvin around becomes important. No doubt he'll being looking for a starting gig and I don't know if the Bills will promise him that. But given his performance and statue right now, I cannot believe he'll be expensive to sign into an extension.