I used NetNewsWire back in the early '00s, but it has become a complicated beast, and after learning about Reeder beta 1 a few months ago, I was an instant convert.
Reeder is great, because all I ever do is arrow-down through news stories, glance over ones I might be interested, press 'B' to open the really interesting articles in my browser, and then press 'A' then 'return' to mark all stories as read.
After about a week's use of Reeder for Mac (currently at 'Draft 4' status), I can finally ditch NetNewsWire, and confidently state that I would pay maybe something like $20 for this new Mac application.
NetNewsWire has always been a little slow, a little clunky, but at least it was reliable.
Reeder brings a lot of the simplicity of the iOS Reeder apps (both of which I have - Reeder for iPad and Reeder for iPhone) to desktop Macs. There are some odd UI glitches that are simple annoyances, but don't get in the way of my personal use of the app.
However, every new draft release brings with it more goodies, more polish, and more awesomesauce.