My team has been using HttpWatch for a few months now to analyze the client side performance of SkyDrive.  Previously, I had relied on some internal tools we use that are based on NetMon, as well as Fiddler and Firebug with the YSlow plugin.  While these tools were pretty good, here's why I think HttpWatch beats them all:

• Great UI.  I haven't used any other tools that combine a detailed grid view with a timeline like this. In one glance, you can see where your time is spent, down to the individual http transfer

• Supports tracing SSL page loads - about 2/3 of our pages on SkyDrive are secure https, and many tools aren't smart enough to decode the encrypted information.  HttpWatch works just as well with https as it does with http
• Automation support - our testers have integrated HttpWatch with our automation framework, so we automatically get full page load traces for any failures.
• Shows details for each transfer.  Breaks down each file transferred into DNS lookup, Connect, Send, Wait, and Receive times, and also lets you look at all the raw HTTP headers cookies, etc.

• Great external app for opening a bunch of saved traces and doing comparisons.

In all, I love the little app, and have only a few feature requests

• Firefox support.  I use Firefox as my main browser, and it would be nice to have the same load time measurements from two different browsers
• A little more documentation on how all the magic actually works.  Is HttpWatch secretly setting up an HTTP proxy like Fiddler, which might change page load behavior?  Is it using a filter hooked in to IE's pipeline?  What do all the "blocked" regions in the timeline mean?  Is it looking at the Document Complete IE event to tell where to break up the different page loads?  I haven't been able to find any of this out with some basic searching online.
• WAN simulation - we use an internal tool to slow down our insanely fast gigE network to realistic broadband speeds for testing in order to get realistic end-user numbers.  This would be a great addition to HTTPWatch.
• YSlow-style grading.  I like how YSlow has an algorithm to rate your site.  Even though no algorithm will totally capture all elements of how fast a page is, it is nice to be able to roll up the performance of a site into one number that is independent of number of seconds taken.