I look forward to at least trying it...it uses TeleNav which I like...I like Google Nav but it's a huge app in the phone...and like most GPS applications...it can get funky in the "last quarter mile"...it missed a store destination for me yesterday by well over 1/4 mile. There aren't a lot of new roads where I am now, but Google Nav, to my surprise, doesn't seem to be that updated maps-wise where there are in fact new roads.
And hopefully as this is a 1.0 beta, they will improve it quickly...we shall see. One other thing Mapquest is touting is the fact they will always be free...Google Nav hasn't done that to my knowledge, and they have been known to kill off apps once they've finished the Beta and learned whatever they wanted to.