I had previously been using Google Reader for web-based RSS reading. I don't run a desktop client because I am on far too many different computers to keep a desktop client in sync. I am also a contractor (for about another month) so don't want to have to install applications to keep track of the blogs I read.
Google reader is ok. It's pretty fast, but there was one thing that really, REALLY, annoyed me about it. It kept loosing posts, or more correctly, marking old posts as read, even if I hadn't.
Also, I have recently purchased an iPod Touch, and wasn't a fan of the GReader touch application. So looking for something better reader, Simone put me on to Newsgator. I've been using it a week and have become a bit of a fan. The web reader is a little slower than GReader, but has nicer features, such as the ability to tag posts, more powerful "Mark As" features and nicer rendering of blogs being read.
The major winning point is that is seemlessly syncs to the desktop client and the free iPhone application, which is very good too.
So, if you are looking for a replacement to GReader, then give Newsgator a try.