I've used NewsFire, Vienna, NetNewsWire and Opera's RSS feature, in that order of frequency. I just discovered RSSOwl. It works very well, but it has one flaw (unless I just haven't figured it out), which is it doesn't seem to save the feeds. When you close the program or go offline, only the feeds you've opened retain their content. I'm not sure why that is, but it's the only problem with what otherwise is an elegant, speedy, and quite capable program.