I like the idea of saving all contacts. I have lost phone numbers WAAAY too many times! and the idea of editing the calendar on my computer sounds great! I am a shift worker so it is handy having the option of putting that data in the phone months at a time.
I have it on a trial right now and don't think I'll be getting rid of it. I have never lost my phone but for the Find my iPhone feature is almost worth it alone for the 'just in case' times. You can upload pictures and on the 3GS you will be able to upload videos to it and create a password so friends and family can view them. The contacts was easy to transfer to mobileme and that is another feature that is worth it to have. The best part of the contacts on the me.com site is that they are set up the same way as they are on the phone. If you know how to use your phone you will know how to use mobileme.