When I got my iPhone in July I signed up for the MobileMe trial and like a lot others it was extended, in fact I'm still within my trial period!
However, during the trial I've encountered quite a few problems...
Although over-the-air syncing is extremely cool it's not without it's hitches, during my trial I've lost all the contacts on my phone once, had trouble updating contacts details and have been left with one machine that plainly refuses to sync it's calendar anymore, even after resetting all sync data.
iDisk has been really slow via the Finder and also tends to lock my out after a couple of days resulting in a Force Quit of Finder or a reboot. Once it is working it's not exactly fast either, in fact the web interface was quicker!
The only thing I found really useful was the syncing of bookmarks, that's the one thing I'm missing since I made the decision the MobileMe isn't For Me.
I'm currently seeking free alternatives to MobileMe, I'm now using
delicious for bookmarks,
box.net for online storage and I was going to use Google as the central sync point for calendars and contacts by using
SpanningSync (which worked really well) but I've just discovered a free alternative,
Fruux, which I'll be trying very soon.