I'm not a current user of Provia 400X -- but I've just ordered a few (120) rolls to play with, along with Astia and Provia 100.

I've enjoyed searching Flickr for various film types and seeing what people have done with them. Granted, you have to filter a bit for both poor exposure and possible post-production (which could otherwise cloud your perception of the film), but if you take a broad brush, you'll find the sorts of scenes and subjects that work well for a given film