Feedburner is a very, very clever concept. I use it, you should too. If you happen to be blog-prone, of course.
First, they've got stats nailed. That's probably their most notable secret sauce. RSS readers are dead-easy to code, unlike browsers, but there are all manner of ways that RSS feeds get consumed - in Firefox, behind aggregators, by bots. Making sense of feed traffic is deep voodoo, and by gum, I think they've got it.
Second, they've made using their product dead easy. I'm only a few short rungs up the evolutionary ladder from a lemur, and I could do it. Now, with the merest few clicks of the pointing device, I can revel in all manner of information regarding the masses of subscribers who have flocked to ack/nak for their daily dose of (whatever).
And finally, they are right at the heart of the next wave of the media consumption gestalt - individuals, comme moi, want to consume "news" the same way we now consume music - one track at a time. RSS feeds are consumed in a way email is not - they aren't subject to the same filters that sift through spam. A feed is something you choose, and once chosen, it offers you even more choice. The feeds I consume on a daily basis are as a constant cascade of the freshest newsprint, delivered one story at a time. Feed to index to content, in one swoop.
How much more indicative of an individuals preference are the feeds they read, as opposed to the words they search on? It's the ultimate intersection of content, advertisers, and consumers.
It's a swell business plan, run by some swell people, at *exactly* the right time.
So they won't mind when I advise that they clean up their website, streamline their subscription process (SET SHATNER=ON; "Too....many....choices..."; SET SHATNER=OFF), and tighten the way they articulate their value proposition to a short sentence that can be repeated with ones mouth full of food.
Now that delicious and flickr and konfabulator have fallen to Yahoo! Borg, Feedburner remains one of the last bastions of the blogostructure still sailing with an independent hand on the tiller and with its top gallants full of wind.