I would use it for subtitling normal dialogue lines. If used with a light touch, i.e. 3-frame fadein + 5-frame fadeout, the effect looks professional and increases readability a little bit (IMHO). I do it in DVDMaestro now, but that authoring suite annoys me...mostly because I can't use my preferred workflow: SSA->MaestroSBT+VDub->MuxMan->DVDSubEdit.
It's definetely eye candy, but 99% of the DVDs I watch are subtitled, and I guess I've grown picky about things
