Since Google Reader came along, choosing which app to use to handle RSS feeds has been a no brainer.
RSSOwl is, however, a very decent alternative, which I'd consider using if it Google hadn't trumped the market. RSSOwl is all about organization - managing your RSS feeds so that they're easy to read manage and access. Although Google Reader now offers offline reading via Google Apps, RSSOwl has long allowed its users to read feeds offline. In addition, it's a great solution for those that use both Macs and PCs because it works across both platforms.
The latest version of RSSOwl incorporates many important new features that enhance its functionality, such as a full screen mode, support for searches, an improved notifier, and multi-tabbed browsing. You can now add a password to your feeds, although this is hardly the most essential thing you need in an RSS reader. The overall look of RSSOwl is very basic but it keeps things simple in order to make feeds easy to find and navigate.
I still won't be changing to RSSOwl but as a lightweight cross platform solution, it's a very well developed RSS reader.