esper: i don't think there's any settings in the filters that are "best", it depends so much on the images. but i've now done some experimenting with the smoothing filters in g'mic-enhancement and have found some favourites, particularly thin brush smoothing because it's pretty easy to tweak and i think i know what will happen when i change the settings.
i use g'mic for something in almost every image i make these days. i have my favourite filters, one of them is samj:s metallic filter. i've recently started to play with some screen dumps of old collages i made at polyvore.com and the g'mic filters are really useful to me there. for example, here's a cutout portion of a collage that i wanted to make more like a drawing or painting:
it's a rather tricky thing to get the photo part to look painted without getting the background all messed up, but i like what these g'mic filters did:
(g'mic colours-boost/fade, set to boost, then testing-photo comix-metallic grain)
(g'mic artistic-colored pencils-output new layer, set layer mode to hard light before i clicked ok. very useful to be able to play with layer modes and preview in g'mic before you confirm an action)
(g'mic artistic graphic novel)
and here's the result when i just applied some thin brush smoothing:
i also use g'mic for making patterns and since i'm lazy i use the array filters a lot when i make symmetrical layouts.