NetNewsWire RSS aggregator is getting lots of great press lately. A Wired article reports that sales are "now in five figures" and Dan Gillmor features it prominently in his Sunday column for the San Jose Mercury News.
I've been using it for months and it's been great. I've tried several other aggregators but this one has been the most intuitive. It can be a tad slow when updating feeds but I don't think it's the software but rather the congestion I face at the times I usually update.