I bought Mobile Me mainly cause I like paying at once for the whole year vs when I used to pay monthly for exchange. Plus the online storage & gallery.
It's basically just integrated the best with the iPhone.
I tried Gmail at first, cause it's free. But it wouldn't sync the photos attached to my contacts. You would have to manually attach photos to the contacts again thru Gmail in order to sync correctly. I have too many contacts to go thru that and don't have most of the original pics that are attached anymore.
Also Mobile Me sync flawless with my PC and MS Outlook.