UI Designer asks what it is that makes RSS readers so compelling. He gives is own reason:

"Previously I had bookmarked about 15 sites that I visited on a daily basis, usually in the evening. I'm now easily monitoring 105 sites and it seems to be as easy as navigating to 15."

From a user perspective, this is probably the most common driver for RSS reader app adoption - the ability to consume a (much) larger volume of (substantially more) relevant content.

I have been aware of my own online behaviour changing dramatically since downloading a reader.  Very rarely do I now 'browse' in my browser...drifting around a number of various sites in the hope that something catches my eye.  This is what I did for over a decade.  Today, the first app I fire up in the morning is the RSS reader.  My browser's function has been transformed in its use - from my primary online space traverser to my interaction engine.

UI Designer also mentions that "I now have to remind myself to start my email program which has become painfully overloaded with spam even though I have an excellent spam filer."

The Email app has also changed its scope in function for me personally - from a newsletter receiver and messaging utility to messaging only.  I have unsubscribed (or tried) from every email newsletter I get and switched to an RSS form of content delivery (see Email v RSS, let us move on...).

I haven't come across a comprehensive study trying to understand RSS reader trends and how/if it is changing patterns of online content consumption behaviour.  If you know of one, let me know.

It is still early days in terms of general use, but I'm sure the RSS reader is one of those memes just waiting to hit the tipping point and reach the non-technical masses. Before you know it, my mum might ask me if I've 'subscribed to that RSS feed yet'.