Gotta say, David Tennant drove me nuts, with his fake esturine accent and his constant shouting. As as for Donna - she's old enough to be his mother! Well, not quite, actually, but the actress is my age, and frankly looks it, so it's entirely uncredible that she'd be this ditsy "girl" who lives with her grandparents! I actually prefer the current incarnation of Matt Smith with Amelia Pond as the Dr.'s side kick. Although to be fair to David Tennant, he's a brilliant actor when given something a bit more meaty, like Shakespeare.
Helen - do you guys get The Sarah Jane Adventures on kids TV? It's another spin off but for younger kids starring the side kick from my youth, Sarah Jane Smith. It's a brilliant series, definitely worth letting the kids watch, although very sadly the actress that plays Sarah Jane - then & now - died suddenly a couple of months ago. British pre-teens all over the country are in mourning for her - spurred on by the BBC!