I had a few problems with NetNewsWire lately. For one my computer became unresponsive while NetNewsWire was updating all my feeds. Additionally there seem to be a few memory leaks in the application. The problem with the computer becoming unresponsive seems to be resolved now, after I deleted the folder ~/Library/Caches/NetNewsWire as suggested by support.
But I still took a few minutes to investigate other feed reader on OS X. Seems there isn’t all that much of choice in that area. I quite like Vienna. Though I found two big blockers for me. It seems to always sort feeds and folders alphabetically. Also it doesn’t provide per-feed refresh settings. I do sort my feeds manually as a way of priorizing them. Also I have a few feeds I use for work and I want to have them updated hourly (actually I’d like to have them updated more often, but NetNewsWire doesn’t permit this). But should NetNewsWire ever have a paid upgrade, I’ll probably have a hard look at Vienna (and it’s source code if there are a few things I don’t like yet).
I also tried NewsFire. But somehow during the first two minutes of using it I didn’t get a handle on it, so I stopped using it again. It costs money and it seems that you can’t try out the commercial version but only the light version before purchasing. A big no-no for me.