I’m sorely disappointed with MobileMe.
First, rules are not available on the webclient. So, if you have any sort of filtering (mailing lists, loved ones, etc.), that has to be done with mail.app. That means a message is received in the cloud, pushed to your Mac, processed my mail.app, then uploaded to the server, where it is then reflected across the wed interface and your iDevice. Since I leave my laptop in sleep mode when I’m not using it, I get a lot of mail in my inbox that would normally be filtered into folders.
Next, although you can specify a personal domain for iWeb, you can’t do so for for email. So any email I send from the web interface comes from <my.name>@me.com instead of <my.name>@<my.domain>.com.
Next, there isn’t any sort of intelligent adding for calendaring. Gmail allows me to add an event by typing “Dinner at Joes on Friday” and it will parse and figure out the correct date.
Performance is also dreadful, though that may be growing pains.
Finally (and I’m not a hundred percent sure about this), I don’t believe there is any spam filtering on the web client, so you have to leave mail.app to handle this. I get a lot of spam (though I’m using gmail to filter most) and I don’t need that pushed to my iPhone.
Overall, unless there is a lot of improvement over the next 60 days, I’m not going to be subscribing. I’ll stick with gmail and Spanning Sync. It doesn’t have the push capability, but it syncs calendar and contacts. I’m also willing to bet Google (or a third party) will be coming out with a similar push capability once Apple opens up push to developers.