The biggest problem: there seem to be almost no members. Without members, it's useless.
Keynote, or Webperf, are basically services that will load urls from many different sources, or agents, and then report the figures back to a central server that gathers them and figures out things like average response time. It's very useful for getting a real view of how fast your site is responding from actual real-world locations.
The more agents that are running, the more useful the results are; with a single agent checking your url you're basically entirely dependant on the speed of their system; with 10 it'll remove a lot of the error caused by problems on the client-side, and with 100 or 1000 of course it continues to get more accurate.
Webperf, as far as I can tell, seems to have enough users to check each url approximately twice in a given time period. That's bad. Keynote, a paid service, can check each url you give it from hundreds of agents every minute, so you get a pretty good view of what's going on.
If WebPerf can get a larger userbase it could be really useful, but right now it's not looking too good. I'm not sure why. If anyone out there wants to join as well, it'll make it better for everyone. (Hint, hint.) ;)
(Posted with Drivel.)