I'm experimenting with multiple ways of measuring this blog's traffic (although that could be compared to hooking an EKG up to a dead body.) I've used Google Analytics for some time, as well as FeedBurner's stats. They're both Google services, but they measure different things: Google Analytics measures traffic to the site, while FeedBurner measures subscribers to my RSS feeds. Both are important in figuring out who's using the site and what articles get the most interest.
IZEARanks.com currently offers three metrics: RealRank, Page Views and Unique Visitors. According to IZEARanks.com, RealRank is calculated by comparing all blogs in their system 70% on daily unique visitors, 20% by daily active inbound links and 10% by daily page views. Quantcast tracks number of monthly unique visitors and frequency of visits, but it also attempts to identify the demographics of visitors. Quantcast also has an interesting media planner feature that enables advertising media buyers to select sites based on their demographics, reach and other characteristics, and to compare sites on those same elements.
For now IZEARanks.com seems to be better for "bragging rights" for blogs, while Quantcast is setting itself up to be a real challenger to Nielsen//NetRatings and comScore. Bloggers don't have to pay a dime to use any of these services, so it's worth experimenting with them to find out which ones get you the best information and sales tools.