Google Reader is a good RSS reader. I've used a variety of feed readers over the past few years; NetNewsWire, Bloglines, FeedDemon, etc. They all seem great at first and then eventually they start to suck. Google Reader is just shiny and new and awesome.
The UI is a freakin' marvel. Clean and simple, each story is presented nicely in a box for easy reading. The page is all AJAX up and down but not in an obtrusive way, it just works like you'd want. The UI seems simple, but underneath is a bunch of careful design and technological complications that make it work right. My favourite feature is the way scrolling down in the viewer automatically marks things read as you pass by them. So clever!
The only thing that would make the product perfect for me is a single key to mash to read the next item. Spacebar almost does it, but it has a bug for articles longer than one page (it scrolls down the article you're looking at, but then skips the next article). The j key does always go to next article, but doesn't scroll the current one if it's longer than one screen. And I wish I could go through the articles in my category order rather than the seemingly random order I get with "All items".
But those are minor quibbles. It's a great product in a space that's really needed some innovation. Thanks, guys!
PS: OPML is a good thing too. It's an awful format, but it works well enough to let me painlessly try out different readers. Took me 30 seconds to migrate from FeedDemon to Google Reader.