We use a flow through brush and have great results. FYI we don't run water through it, just dip in solution and go to town
Ditto. Get a car washing brush or the Mr. Long Arm brush (I think they are identical) and put it on an extension pole. The flow through handles that often come with auto brushes aren't long enough to reach really tall gutter. Mix up a batch of gutter zap, gutter grenade, or simple green, dip the brush in your solution and lift it up to the gutter and start scrubbing. I always rinse the gutters with a hose when I'm done.
P.S. I've used bleach and TSP before which works really good as far as whitening the gutters, but the problem is that if you wash the gutters over a wooden surface like a deck, the solution can bleach the wood