Is there a way to measure data traffic from airport extreme? We have lot of machines and occasionally experience slowness in some devices and wanted to see if it has something do with a device stealing the bandwidth. So I would like to measure the bandwidth utilized by each device.

Of course the other reason is our cable company. But I am afraid there will be revolt if I criticize them.
PeakHour on the Apple Store can use snmp to monitor the bandwidth usage of network routers (including AirPorts). I don't know if can measure individual clients though.

Larger scale you could use something like Cacti to monitor all of your devices and graph their statistics such as open connections, bandwidth, etc.