"In the best case, the comet is big, bright, and skirts the sun next November. It would be extremely bright - negative magnitudes maybe - and naked-eye visible for observers in the Northern Hemisphere for at least a couple of months," Karl Battams, of the NASA-supported Sungrazer Comet Project, told Spaceweather.com.
However, Battams concedes that this outcome is far from certain. "Alternately, comets can and often do fizzle out! Comet Elenin springs to mind as a recent example, but there are more famous examples of comets that got the astronomy community seriously worked up, only to fizzle," he said.
In a guest blog for the Planetary Society, astronomer Bill Gray agrees, pointing out that Comet ISON's orbit has been very well constrained, but just how bright the comet will become is anyone's guess.
"...estimating comet brightnesses a year ahead of time is about like asking who's going to win the World Series next year," writes Gray. "It could be astonishingly bright, or it could fizzle. I think it was David Levy (co-discoverer of Comet Shoemaker–Levy 9) who said that comets are like cats: they have tails, and do whatever they want to do."