I love the rest of MobileMe, but the Web Mail is dreadful. It is incredibly slow, and if you leave it open for any length of time, it seems to time-out, leading to two annoying scenarios:
1) You go to click on an email in your inbox, and the page refreshes, making you click it again
2) You finish typing a long email, and when you press send it tells you it's lost connection and the email and it disappears (I always command-C my emails before I press send because of this).
I am a freelancer who often works on different computers, and I need the webmail. It drives me nuts. My old GMail account is so snappy and reliable in comparison (although if they could only speed up MM the interface would be fantastic).