It depends on the zoo. For the most part, I think they are a positive thing. As long as the animals are well cared for and as happy as possible. Most zoos have a goal to educate the general public about endangered species and animals in general, which I think is a positive thing. Many of them also offer scientists a unique opportunity to study wild animals that would be very hard to learn about outside of captivity.
I think, as far as conservation goes, that some zoos do a good job working with endangered species. However, the sad fact of reality is that unless things change drastically, many of these species already or very soon will no longer have a place where they can survive in the wild.
Then the only places some animals will be able to survive will be in zoos and similar facilities. Personally I'd rather see species live on in captivity than see them go extinct. But that's just my personal opinion.