First off, yes there's a little ad in the corner, unless you pay. Personally, I find it pretty easy to ignore. YMMV.
Secondly, I was one of those suckers who paid $30 for the 3.0 upgrade, and I was a bit upset when I was told I needed to pay another $15 to get rid of the ad in 3.2; that's kinda cheezy for a minor point release, and I haven't paid. Like I said, I can live with the stupid little ad. But it's easily worth $15; hell, it's easily worth twice that. Which is what I paid already, hence my complaint.
But those two nits aside, NetNewsWire is the BEST feedreader on the Mac - and in my admittedly limited experience, any platform - bar none. Nothing else even comes close.
First of all, it's one of the few on the mac to use Google Reader to sync. That means I can read my feeds at work, at home, on my iPhone, and they stay in sync. I never have to seen the same article twice.
The interface is sleek and fast. Using just the arrow keys, I can blast through hundreds of articles in dozens of feeds, opening the interesting ones in tabs or my browser for further reading, in about a minute.
It's also extensible with scripts; I use the Readability2 script to reformat articles for easier reading, and others to send articles to Instapaper, EverNote, Twitter, Tumblr, and more.
The preferences give me a lot control over the appearance of feeds, articles and web pages; my work machine is a 24" iMac, so it's nice to be able to specify minimum font sizes for legibility, and I can set it to refresh just once an hour, when I take a five-minute break.
So ignore the haters around here who just can't standa little ad banner, or think that $15 is too much to pay to support a best-in-class application. NetNewsWire is the Real Deal. Accept no substitutes.