There's not been much written about User Engagement with RSS Feeds - today FeedBurner published a study on it which I found out about from TechCrunch. I'm not sure "Engagement" is the right word for what this study shows.
Google Reader - 59% of all the RSS traffic that Feedburner tracks/feeds goes to Google Reader (see below):

Click-through is actually meaningless since you can read an entire feed in Google Reader while you can't in MyYahoo.
Feed Diversity

Also, 76% of all the RSS Subscribers were initiated by Google Reader (or Google Personalized homepage, I guess).
I don't really see the "engagement metric" here - I'd call it something else - maybe what reader users would rather use.
Interesting that Feedburner published this study - because it has the data to provide these numbers, which are very likely accurate.
