I think overall that the finale episode(s) were very good. There are always some bits from Doctor Who which are likely to annoy people, and that's especially true in this sequence of episodes, that contained an awful lot of stuff.
The towing of the Earth back into place was a bit silly, and K9 also annoys me, since even by the standards of the era when he first appeared, he's a fairly naff special effect.
There were some great bits in there though. The Harriet Jones bits with the daleks; "Harriet Jones, ex-Prime Minister", "We know who you are <zap>" was a classic, and Doctor Donna was very amusing, as was the bit with the second Doctor acting a bit Donna-ish.
Over the series I've got to quite like Donna, so it'll be a bit of a challenge to find someone as good to replace her. Billie Piper as Rose was frankly awful in these episodes, which was odd, because by the time she left originally, the character had become rather good. Freema Agyeman as Martha Jones was also a bit meh. I've never found Martha to be as interesting as either Rose or Donna, but I think that's possibly the fault of the character more than the actress.
