They are! They have a wonderful rapport and chemistry and it only gets better as the series continues. Donna has gone from making me think (I started watching mid-third series and spent a month or so catching up before I could watch the eps as they aired), "Oh, thank god this Martha Jones exists so I already know for a fact that Donna won't be inflicted on us after this," to making me squee, "GO DONNA GO WOO DONNA." She's dangerously close to overtaking Martha as my favorite companion, and I never thought THAT would happen.