Better than tools.pingdom.com in that it loads all the javascript and code form the site.
Next features that would make it very useful to me are (in order):
- Display timings on the timeline (in Chromium I can't see them, maybe display them on click or hover)
- Recurring checks (of course this is your core. You are already working on this I imagine)
- Different locations in the world (including being able to slice up my reporting based on the location)
- Custom alerts on specific urls (url X cannot take more than Y seconds to load inside my page, beyond more classic ones like total page load time and such)
- Hot cache-Cold cache
- In case of alert also generate a tcptraceroute and compare it to one that is collected every X minutes.
- Ability to set the host header separately (so I can use the IP address in the site url and the host for a specific virtualhost, this is useful when a site is geographically distributed and you just want to cut out the DNS lookup).
Also have a look at many of your potential competitors like Gomez.