You notice it when you've had it, then it's gone..
I remember the first time I saw someone in a dim-lit cafe a few years ago using an PowerBook G4 with a backlit keyboard. My reactions were firstly "wow, what's that laptop? it look cool"; secondly "that looks damn useful" and thirdly having found out what it was "that's way too expensive to be worth it..."..
For the last 2 half years I have owned a 13" MacBook Pro Unibody, with backlit keyboard. I guess I thought I didn't use the backlight a lot, or rather I thought I didn't, until I started carrying around an iPad 2, and Bluetooth keyboard.
I got my MacBook when I returned to full-time education. Switching to an IPad and wireless keyboard to weight sake I have found it amazing when working in cafe corners or a library or a lecture theatre or whatever, how I suddenly realise I can't see my keyboard and how much I miss the backlight.....
I think because the way the backlight works, switching itself on and off intelligently and progressively. The subtle engagement of this function is only realised when it's not there anymore. So I think the backlit keyboard is a big deal, especially on a mobile device that you are going to use in less that perfect environments, or places where you can't control the absent light level.