Alright, it's been 2 weeks since I got the update and here are my observations - Energy monitor seems a bit slower than usual in updating the instant fuel economy gauge - Accelerator pedal seems less sensitive than before - Fuel mileage appears to be unaffected - My dealer said another of their Prius' ECU died while doing the update and they refuse to do another until Toyota tells them why the ECU is prone to self-destruct when they're trying to update the software.
Mine was done on Dec. 14 and I have not noticed any difference. I got my notice in October and waited until they had done several before taking mine in.