Brandy, there is a difference between a bog and a bog filter. A plain old bog can be a muddy, smelly mess.
But a properly installed bog filter will put clean, fresh, healthy water into your pond, and give you beautiful, healthy plants and shallow water for birds and toads. In fact, it can be used to filter your entire pond.
It's all a matter of plumbing. Read that link above, from Nelson's Water Gardens. They use bog filters to filter ALL their ponds, and their place is gorgeous and perfectly clean - even the one that's way overstocked with fat, fat koi that get fed a zillion times a day by visitors plunking down 25c for a handful of feed from the machine.