It was an interesting conclusion to the alien invasion storylilne. There was some really lame stuff, like about half the attempts at humour, especially those by the actors playing the disguised aliens, but there was some stuff that worked, and it was a very bold and outrageous plot with some nice pseudo-science.
The aliens were also kind of neat. A good updated version of what you'd expect from WHO, kind of goofy looking, but nicely designed. Kind of looked like what I would imagine an Alan Davis drawn WHO alien would be.
I also kind of like that they put up some real websites for the ones that are part of the plot.
(the UNIT press release on the file sharing virus is cute)
Next episode is a familiar "face", really good preview.