My biggest gripe with RSSOwl was that it was that it had some performance issues, especially if you had a lot of feeds. And using Google Reader eliminated the need to synchronize my list of feeds between multiple RSSOwl instance (like, say, my personal laptop and my company laptop).

But the first issue may be mitigated by hardware improvements since the last time I was using RSSOwl regularly, and I have some ideas around the second issue.