I was VERY underwhelmed with David Tennant's few seconds on screen after Christopher Eccolstone regenerated. But once the series started with him in the driver's seat, he proved me wrong.
Not only do we get a mere few seconds of him in the role, but clearly no-one has sat down and thought about how the new doctor is going to be as a character. You can deduce that from the fact that different writers are going to be making that change.
In fact, many actors require a few episodes to really bed in to the part and then become great. Heck, look at someone like Patrick Stewart, as Jean-Luc Picard, on Star Trek: The Next Generation. It was probably a year before he really fitted into the role and then he became spectacular. When you consider things like that, it's hard to honestly make a judgement based on a 20-second sequence.
He definately deserves one whole episode and preferably deserves the whole next series, before we start passing an honest judgement.