Doctor Who: change of characters?

I just started watching the show (the new one), and I really like it. I'm approaching the 2nd series finale, but I'm avoiding it because I know Billie Piper gets replaced, and I really like her chemistry w/ David Tennant, and I've seen the next girl and she doesn't look as fun. Oh and I've also seen pictures of Matt Smith, and I don't want to watch him either! Please tell me something good about the new characters...