Currently working on a cloud platform for an ISP and found a great command line tool for your ESXi hosts to see how your storage is performing. SSH into one of your servers and run ESXTOP and press the D key.
This shows usful information like how long a storage command has spent in the kernel v's the amount of time that it has spent going to the device.
See link below for further information:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1008205
