I own Filterstorm Pro and consider it in my top 5 best apps for the iPad. It has an intuitive interface that makes users of Lightroom or Aperture feel at home and all of the editing features you could reasonably expect. My only real qualm with it is the workflow of getting your edited photos out of Filterstorm and back on a computer. The various methods of transferring them currently are clunky at best and nonfunctional at worst (when working in bulk).
For me personally 99% of the time I don't even need to do much "editing" of the photos on the iPad. The thing that is useful for me is the ability apply metadata after a shoot. It's handy to browse through a large number of images, delete the bad ones, and tag star ratings to the keepers. This saves me time as I don't have to do that again in Aperture later on. I'd really like to have an app that applies the metadata directly to the photos on the camera roll (or in events) rather than have to "import" photos into Filterstorm then "export" them back out.
Actually, if Apple would just allow star ratings to be applied in the built in Photos app I'd be almost perfectly content.