I think it will be great when it works, but right now it's pi##ing me off.
When I signed up to it and set it up on Friday, everything worked pretty much as advertised. If I made a new event on my iPhone, in about 10-15 mins it was on my mac and vice versa. For some reason now that's completely stopped working. In fact, the syncing for everything apart from Mail isn't working OTA which is kinda annoying, but it's just one of those things you know they'll fix very soon.
I haven't used the @me yet because I have a gmail account, but I imagine over time I'll use .Me for more serious stuff. Also the filehosting thing sounds nice, but Rapidshare kinda ties me over for that.