Ok, pretty poor. Trying to work out why, but definitely in contention with Fear Her, Next Doctor, Planet of the Dead and the Rise of the Cybermen pair for weakest episode since the revival.
In part, I think it's that it was the modern day version of the bolting-together-Who-cliches stories approoach that produced Timelash and Warriors of the Deep. Then it was an old friend of the Doctor's who been mutated and become vengeful, or a magic chemical that could kill the monsters and gets mentioned in the first scene; now it's a child, some sort of appeal to the threat, and so on.
Not a Time and the Rani, Threshold, etc, level of bad, but also... only a few steps above TOWIE as a waste of airtime.