Byline, as you might expect, fetches all your subscriptions from Google Reader and displays them for you to read. It also keeps track of the items you've read and starred.
The greatest thing, though, especially for iPod touch owners and subway riders is this: Starring or Noting an item saves it on your iPhone or iPod for offline reading. If you star an item either using your desktop browser, Mobile Safari, or Byline itself, it saves it for later. Noting an item is even better, because you can save pages that aren't even in your RSS subscriptions for offline reading with Byline. They mention you can do this very easily with a bookmarklet.
Now -- I can hear you screaming: NetNewsWire does all this, and it's free! Without boring you with detail, it's worth ten bucks to me to keep my Google Reader workflow the way it is, rather than spending time changing the way I work. Agreed: it's probably too steep for people who aren't heavily invested in Google Reader already.
Additionally on the downside, Byline doesn't allow you to browse by tag, instead opting to lump everything together in one list by date. Phantom Fish says this is for performance reasons, as people with many feeds would experience sluggish behavior. They're looking into it, though, and I personally hope they figure it out.
Byline is $9.99, awesome, and available through the App Store.