I used one for awhile that came with a tank that I bought used on CL. Not a fan, personally, for a number of reasons: media choices are limited, at best; the canister doesn't hold much media for it's size; servicing is a pain compared to other canisters; not as easy to restrict the flow or use a spray bar.
I actually worked at, and was eventually a partner in, several LFS back in the day, and used to service customer's tanks as well as the ones in the store. Always hated Magnums, compared to Eheims and Fluvals. Never would've had one on a personal tank if it hadn't been part of the package deal. Replaced long since. (Tank happens to be in daughter's room, so I went with dual AQ50s to make life easy for me, and eliminate the possibility of her disconnecting a hose on a canister and draining the tank! Still prefer canisters overall.)