I have mixed feelings about it and agree with Lillyleaf that it is realistic... for Hollywood!
I do not believe at all that it is realistic in terms of the real life DID experience however, and the show often irks me for this reason. It doesn't help the general population understand DID - and even perpetuates many myths about it - but then I suppose its purpose is to entertain and not inform, so what can one expect.
Most people with DID do NOT switch clothes when they switch internally, and neither do most alts have distinctive accents etc. In fact many times individual alters go to great lengths to not reveal their individuality to outsiders, and only those very close to a DID person may know a switch has even occured. But obviously they needed to make the alters very disctinctive on the show for the benefit of the viewing audience.
I do like watching it though. I think they've done an imperfect but passable job of mainstreaming the topic.