I have the Kobo Libra Color and the contrast on the X4 is much much better.

The whole sticking-it-to-the-back-of-your-phone i find a bit useless, but mine came with two metal rings so that I can stick it to other places.

I have one ring on the dashboard of my car so the X4 can hang there until i need to use it. Good for trips with many stops.