I've started to use the RSS feeds, and they are great. The problem with the discussions feeds is that I don't know when there is a new addition (My RSS reader marks the read links) to the thread. A possible solutio is to change the url everytime a message is added (for example by adding the number of replies, see http://www.joelonsoftware.com/articles/BuildingCommunitieswithSo.html
Good idea David. We use this idea on the main pages where the url has article_count=X. So, if someone adds a message, the browser will show the link as "unread" letting you know that something is new. We should add this idea to the RSS feeds and will do so.
We are also working on email integration where you will be able to subscribe to forums and threads.