We bought ours equipped with the nav/energy management display to get the hybrid displays, as well. Both my husband and I are geeks, and we wanted all the features the hybrid came with if possible -- especially if they were absolutely unique to the hybrid. The price was painful but we figured we'll have this vehicle a long time and want to enjoy it and show it off, and those displays make it a lot more interesting chatting with folks in the passenger/back seats about how the hybrid functions. "See the arrows?" "Ooooh! Your engine just went off! You're electric right now!?!"
I can't say what I think my MPG would be without the screens. Who knows. But I know I always have those screens on, so much so that I wish you could program one of the hybrid energy mgmt. screens to come up at vehicle start-up, instead of your radio/CD/etc.
(Not sure about the anti-portable GPS vibe I'm getting. I use one, and leave my FEH's nav screen available for the hybrid energy displays, thereby providing me with fuel economy data *and* nav capabilities without interruption of either.)