I think pp, above, may be correct that, like a lot of classic Who companions, most NuWho companions have good first episodes but then don't necessarily capitalise on their potential. Martha definitely suffered from this and, I think, Clara also - though the character seems to have been salvaged rather last season (I think anyway).

I think its a very difficult line for a show like Dr Who to judge - just how much people will swallow. There are always people, like a friend of a friend I once met who objected to a Robin Hood adaptation because the minarets in Jerusalem in the first scene were incorrect (and you are never going to win over those people), on the other hand there can be a moment when whatever it is you are doing stretches most of the audiences credulity too far. I think both Smith and Jones and Gridlock are just on the right side of that line for Dr Who, but they are both very, very close to it.