My opinion might not be a popular one but......
I don't like or use canisters. I'm not disputing that some of them do a decent job, thier just a pain in the ass. Most of them are not much more than and expensive, pain in tha ass to service version of a hang on the back filter.
A hang on the back filter does what? Traps dirt in a sponge or pad, has activated carbon for chemical filtration and many have a devise or area for biological filtration. (Most if not all are lousy at biological filtration) To service a hang on the back, you reach to the back of the tank and remove the filter sleeve / media and rince or replace it as needed. Most aquariums need thier "dirt trap" rinced off one or two time per week to keep the aquarium optimal and carbon replaced about every 4-6 weeks.
Canister filters do what? The exact same thing a listed above except some move more water per hour and may have a larger dirt trap and hold more carbon & biological media. When it comes time to service them though, it's not a easy as the hob. How many people take thier canister apart once per week to rince the filter pad / sponge? I bet not many, it's a pain to do. Most canisters also (just like hob's) don't have a large enough area for biological media for the average aquarium. I bet two aqua clear 500's would perform as well at trapping dirt and chemical filtration as the fluval 404 for less money and a ton easier to maintain..
I use wet/dry's on just about all my tanks I own or maintain. The prefilter can be rinced in a minute or less, the carbon can be replaced in a minute or less, it's biological capasity is far superior to canisters & hob. THey are much cheaper to maintain because you don't replace anything in them (except carbon).
Don't get me wrong, I'm not saying it's wrong or foolish to use a canister, some of them are well designed units (ehiem for example). I want ease of maintenance with excellent performance and canisters don't meet that need for me.
Joel