The crap plastic alien was mostly off screen and wasn't terrible (maybe Classic Who has tempered my critical faculties a bit here). I thought the acting was fine - Catherine Tate was less strong than she's sometimes been but partly that was becauae this was an earlier, more shouty Donna.

Filler, yes, but it did at least tell a self-contained story, albeit one that's mainly set-up for a different story. Rather like Utopia in that respect, only without the pant-wettingly cool bit at the end.