I purchased Feeds because I thought it would be a good way to keep up to date with several activity feeds that are important for my work: Github and Basecamp. I'll skip the part about it not supporting Redmine or generic atom feeds because that's being discussed elsewhere.

With just my Github and Basecamp feeds set up, Feeds broke down as a useful tool. It only shows the last N updates. My Github and Basecamp feeds are pretty high volume, so this guarantees I'm going to miss something once one or the other refreshes. Once I realized that, I knew I had to stick with my current approach of adding RSS feeds to Mail.app

What's become clear to me is that Feeds is an app that wants you to be there monitoring it frequently. That's not how I work and I can't imagine being productive with the interruption of a feed monitor. it might increase the stickiness of user engagement but at the expense of the user's productivity, concentration, and attention. I contrast it with something like Socialite and it's pretty stark. Socialite will show me all the activity but it's on my own terms and not a click-the-menu-bar-icon-or-risk-missing-something-important situation.