I was a big-time Discus user back in the early 2000s. With some tweaking, it was very SEO friendly. Then, Discusware did a major "upgrade" that would have changed my URLs and SEO strategy. Instead of implementing the new version, I switched to vBulletin and never looked back. It was an interesting product for low and medium volume forums. One cool feature of the original version was that "forums" and "threads" were kind of similar. You could start a thread within a thread, and in essence users could create new topical forums. That could get messy, too, of course, but it was a great way to identify topics that people wanted to talk about - those topics kept getting posts and would stay near the top of the list.
I didn't find Discus to be too similar to BestBBS, which is unique to WebmasterWorld. BestBBS is far more scalable in terms of handling high posting and reading volume.