As Woody noted, they're really nice when you need them. I don't use them on every model I build, but find that they will often get into areas that I can't get sandpaper or straight files into with out damaging the surrounding details. Plus the shapes often let me maintain curves on the model better than straight files or sandpaper.
Having said that, would I recommend you run right out and by a set of riffler files. That depends. If you already own a set of jewelers files, then by all means get the rifflers. However, if you don't have a set of jewelers files, I feel your money would be better spent getting them first, then a while down the road, getting the riffler files.