RSS reader support has been a built-in feature of Mail.app since version 3.0 (when Mac OS X 10.5 Leopard was released). Although I subscribe to a number of RSS feeds, I've yet to find the need for a dedicated RSS reader app -- such as Newsgator's NetNewsWire.
I like having my email and news feeds in the same application. By default, RSS feeds that you subscribe to in Mail will appear in their own folder. If you'd like your RSS feeds to appear in your inbox, just click on the upward pointing arrow of an RSS feed(s) within the RSS section in Mail.app.
On the downside, this view of RSS feeds may flood your inbox and be overwhelming, depending on the number and the activity of feeds you're subscribed to. As a result, you may want to be selective in choosing your feeds. However, I've found this view particularly handy with time-sensitive feeds, and have used it on numerous occasions to score in-demand concert tickets (at face value prices, too!) with search-based Craigslist feeds.