It is mainly my experience through the browser. Whenever I get on it at different places I have worked, or through other people's machines via Safari, Firefox, Opera, and IE, I get issues.
It won't refresh when new email arrives. It will error out on a 2nd or 3rd email I am trying to send. The page will load blank. It is slow as all get out. It logs me out randomly and it never, never NEVER pushes. I compare all of this to my experience with gmail, which has its problems too, but runs great. It is also free, which makes my paying for MobileME, ridiculously painful.
I would say all of my issues appeared last iPhone launch cycle with the 3G. remember the outages and the nightmare Apple created by too many variables on the same day. Software updates, infrastructure shift, online service swap, account creation, billing, and hyped product launch all in a 4 hour period. Nice work boys, you had a melt down, and I feel I am still dealing with the side effects.