Mars Under Fire
Scientists appear to be pretty excited about the possibility because an impact would eject material from deep under the surface and give them a chance to take readings from the orbiting probes. In all of our missions to Mars, none has ever done more than a scratch on the surface analysis of what's there, so we have no idea what to expect to see if the asteroid hits.
From the information that's been released so far it isn't clear what side of the planet the impact is likely to happen on. I'm assuming that since nobody has said we can observe it that it will be on the opposite side from Earth. If it is going to be visible though, that would definitely be a cool sight to see. It's not every day one gets a chance to watch an asteroid impact a planet firsthand. And it's certainly not something we'd like to observe firsthand here on Earth.
NASA says the Opportunity rover is in no danger since it would be outside the projected impact zone. That also seems to imply it wouldn't be practical to drive up there to check things out afterward either.
RIP United States of America
July 1776 - November 2012.