The Mail application is easy to set up, integrates with my other email addresses and gives me a @me.com email address too. It acts a lot like Apple Mail, with just a little less gray (which is okay with me). If you like Apple Mail on your desktop you will probably like Mail in Mobile Me. You do seem to loose the ability to create an event directly from the email message (at least I haven't been able to figure how).
The Calendar function acts a lot like its desktop counterpart, there are some differences. I found out that you cannot embed your Mobile Me iCal calendar in a Google site, it is an unsupported calendar. Not being able to this, kind of defeats part of the reason that I wanted an iCal online calendar - oh well just have to keep gCal synched in the mix. The Calendar application it works quite well and is easy to use.