Performancing Metrics (aka pMetrics) Gives you an amazing amount of detail about the visitors to your site. In some cases, it's the same data you get from other packages, but it's easier to get to and understand in pMetrics. In other cases, it's the kind of data you don't get elsewhere (easily).
Here's an example of a piece of some interesting information I gleaned from pMetrics. I run a candy blog and many times when we review a product, I have a contact with the company already and I email them when we write about one of their products (you do this every time you mention or review a product on your site don't you?). Recently, we reviewed a product and I had no contact with the company so I went to their site (let's call it abc-company-xyz.com) and found a generic contact form (no actual email address), so I filled it out letting them know we had reviewed their product.
The next day, I'm looking at pMetric's Top Referers for the day and I see a link from mail.abc-company-xyz.com and when I click on the pMetrics link, it shows me the full link is
http://mail.abc-company-xyz.com/exchange/sue.smith/Inbox/Candy+Addict+review+of+our+product.EML?Cmd=open - what do I get from this? My email got routed to someone named Sue Smith (name changed) and she can more than likely be contacted at sue.smith@abc-company-xyz.com - I now have an email address for someone inside the company and I can email her directly now.
Another example of email-related info I get from pMetrics: I can see what links to my site that people are emailing. How? On the Top Referers page in pMetrics I see various links from webmail sites like by102w.bay102.mail.live.com, us.f820.mail.yahoo.com, webmail1.webmail.aol.com, etc. Click on the link and you will see the visitor's info then click on the actions link and the first action (their arrival on your site) is the link that was sent in an email to them.
To see even more, change the url in the browser on the page that says "Visitors from us.f820.mail.yahoo.com" and remove the beginning subdomains (us.f820 in this case) so the URL just has mail.yahoo.com in it and you will see all emailed links that were clicked from Yahoo mail.
You should realize that this doesn't show you every link to your site that was emailed - only the ones that were actually clicked in a webmail application.