I use Fiddler all the time to debug web application. Its an awesome, awesome, awesome (did I say awesome) tool to debug all sorts of issues including http request/response, peformance problems, JSON etc etc. Sometimes I also used IE developer tools and always wondered about what are the use cases for using IE dev tools vs Fiddler. Some differences are quite obvious, once you start using these tools however others may not be too obvious. A few months back, I came across this presentation by Eric Lawrence at channel 9. In this presentation, Eric used following slide to provide comparison between IE dev tools and Fiddler .
I thought you may find it useful too. Until next happy debugging.