I’d been struggling for a while to find a decent RSS reader for Windows. However, I’ve now been using Feedreader for a few weeks, and am very happy with it. It fully supports nested folders/categories, which is nigh-on essential if you’re regularly monitoring as many feeds as I am (>100). You can effectively aggregate several feeds together by viewing them at the folder level. Feeds can be viewed using the text contained within the feed itself, or you can easily open the original blog entry inline. The OPML import/export support seems robust, and fully supports the nested folders. Feedreader will also discover feeds in a relatively intelligent way if you feed it a blog URL, as well as supporting searching across all cached blog entries.
All in all, pretty impressive.