DropBox is a very good one trick pony. If all you want is some online storage it works at least as well as MobileMe's i'Disk, and if 2GB is enough then it's free.
MobileMe does a lot of things: In order to get almost* everything it does you'll need a few other accounts besides DropBox; Mail, Calendar, Contacts, Gallery, Website hosting, and a few miscellaneous others.
However MobileMe does cost money, at least for now. All those other things can be gotten for free from services like; Google, Flickr, Yahoo, etc.
* If you have a Mac, then you get extra syncing features and a few other handy tricks that can not be easily duplicated by other services.