Quite useful to keep track of long chain of emails. Easily find out who sent which message, when, and learn at a single glance if there were some messages in the conversation you didn't receive.
This really clever plugin will really help you sort complicated stuff faster.
It won't be useful if you only deal with single messages with only few replies however. And the disk/memory usage is a bit unclear.
It also still needs some fixes, but it is an extremely useful addon that should be integrated in most webmails and email clients in the future IMO