Are they good or bad? That depends. And that's the answer.
But seriously... why does there have to be a correct answer? If someone is using a canister, and it's working for them and their tank is thriving, then why change? And if someone isn't using one, and their tank is thriving, then why change?
Canister filters equal more water volume... that's a good thing. It gives you a place to run chemical filtration... that's a good thing. It gives crud more places to get trapped and decompose that a cleanup crew can't reach... that's a bad thing. It is one more thing to leak or generally make life miserable... that's a bad thing.
If you keep up your maintenance and clean any sponges/pads in the things weekly, canisters can be great additions to a reef. If you're lazy on your maintenance, canisters can be a very bad addition to a reef.