Today at the Google Reader Blog, Mihai Parparita discussed the new way Google will be calculating feed subscribers.  This was in response to many people noticing huge fluctuations in their RSS subscribers counts.  Mihai says there is a difference between how subscribers are calculated at Google compared to FeedBurner.

Google subscriber counts includes all subscribers to all of Google's services including Orkut, Google reader and iGoogle where as FeedBurner only counts subscribers to feeds which are redirected to Feedburner.

Google subscriber counts: These numbers include subscribers across all Google services, including Reader, iGoogle, and Orkut. You can see them in Reader's feed search results (pictured below) and the Google Webmaster Tools. Additionally, our crawler reports them to the publisher each time we fetch the feed. Reader's feed search was recently showing stale and incomplete data, but as of today (October 15) the numbers should be the same everywhere.

FeedBurner numbers: If you use FeedBurner to manage and track your feed, you will see a subscriber count there that is attributed to "Google Feedfetcher." This number is a sum of all the feeds that you have redirecting to your FeedBurner feed URL. So if http://www.example.com/atom.xml has 3 subscribers, http://www.example.com/rss.xml has 7 subscribers and http://feeds.feedburner.com/Example (where you redirect the other two feeds now) has 12 subscribers, then you will see 3 + 7 + 12 = 22 subscribers reported in the FeedBurner interface.

Subscribers are an excellent metric in determining the performance of your blog however they are only a piece of the puzzle and I don't suggest you look at subscribers as the only indicator.  Components such as # of comments, pages per visit, time on site, media mentions and authoritative back links are all metrics that should be part of the entire picture.