The first thing I liked about RSS Popper is that it's almost invisible. It only displays three buttons on Outlook's toolbar and the RSS feeds are saved in a folder on the left navigation sidebar, although you'll have to switch to the "All Mail Folders" pane in order to see it. Luckily you can use keyboard shortcuts for faster access.
The program features some default feeds to which you can easily add your favorite ones. RSS elements are displayed as standard messages and managed in the same way you manage your emails. The downside to the program is that it takes quite a long time to refresh and download feeds, so I ended up with the feeling that it was slowing the whole thing down. I also had the sensation that, although having your feeds inside your email client is supposed to be more comfortable, it just feels better to read them online with, say, Google Reader.