I guess i will add my two cents...
I currently have .mac so i will get Mobileme automatically but i am extremely excited about it. Currently, I have 3 computers. A MacBook for School, a MacBook Pro for home and an old Dell that i do random things on in the event I need to use Windows. I also have an iPhone. The advantage to Mobileme is that if I add a contact on my MacBook while at school and then go somewhere with my MBP and i need that contact, it will be there. Basically, if I add a contact right now, I have to plug my iPhone into my computer and sync it, now, i won't have to. The only time I will have to sync it is when I want to add music. All contacts, calenders etc will be synced over the air which will be nice.
For someone like me who used iCal for everything, I find myself syncing my iPhone all the time just so my calender will be up to date. Now instead of daily syncing, i can do it maybe once every week or two just to switch up the music.
This is the main advantage of Mobileme to me and I feel it is targeted to people like myself. Some won't find use for it, but for people like me with 3 computers to keep in sync and an iPhone, it will be a life saver!