There is nothig inherently wrong with canister filters. The only problem is that they tend to trap detritus and then nitrates build up. IF you were to clean the thing out every 3 days it would actually help your tank but most people get lazy and the canister becomes a problem.
I suggest keeping a canister filter around for emergencies, packed full of absorbing media for nitrate, ammonia, copper or whatever toxin. They can really be a life saver.