shaneystar, what I think of as a 'good' adaptation is one that seems to put on screen what I had in my imagination before. That's why I loved LOTR - not because of the money it made but because everything looked just the way I felt it should. Compare it with that weedy cartoon version they did in the 1970s, for instance. As for P&P, there haven't been that many of them but Keira Knightley looked just like my idea of Lizzie, all feisty and giggly, and I thought their use of different houses for different ranks in society - from shabby manor houses up to Burghley - really brought the 18th century alive for me. Much better and livelier than Greer Garson (I never did see all the TV version 10 years back, so can't compare).
There's a rule of thumb, though, that truly good books make indifferent films (Catch-22 or the various War and Peacees, for instance) , while the best adaptations come from books that are okay but not real classics, like The Maltese Falcon. That's because the processes involved in producing, and consuming, books and films are different. But I thought LOTR was an exception to that rule: great book, great film.