I'd have to look at their site again for the details, but my understanding is that is most just gives you SE friendly URLs and may also provide some SEO benefits like dynamic page titles and meta tags.
I'm not sure it's worth the cost, but I don't think there is any "risk" in using except if you have an established forum and it changes the URLs so that your indexed pages are then "gone". But generally the way these things work is that the old URLs will still work, but the links to the on your site will be gone, so in time they will be dropped from the SE, and the "new" URLs will be picked up in their place.