I really like, "MIUI. Love your phone again." Also agree that we should lose unnecessary exclamation marks. "MIUI: Android your way" is great too. They really embody the essence of MIUI. When you've gotten used to all your phone can do... MIUI makes you feel like you just upgraded to a new one. Makes you fall in love with your phone all over again.
This works for those in the know who, well, know what it means, but for those that don't get it...it's like it's so great of a ROM that you'll have to grab the sofa & take a seat to play with all the cool features.
miui, android evolved is pretty legit. the slogan has to be really simple and short; you shouldn't feel like your reading a sentence but rather just a few words. i definitely agree with someone before who said to get rid of the exclamation point tho