The first thing that no-one seems to mention is that MobileMe is a 'suite' of services, not a one-trick pony. You can piecemeal each of the functions of the suite (Cloud disk, IMAP, Sync, etc.) through different providers, or you can get it all in one place through Apple. I think the best thing about this is you have choice, and can make the decision for yourself which service(s) are best for you.
The sync of e-mail messages is part of IMAP, which is available through MobileMe (and a host of other providers, incl. Gmail). Gmail's IMAP implementation isn't pretty but very customizable; it also requires some peaking-and-tweaking to get it to work "just right" with MM. My university uses Gmail (aka. Google Apps for Education) for all campus e-mail. In its default state, it will sync ALL of your e-mail to your IMAP client (incl. the uberfolder, All Mail). This is good and bad--good that you get a copy of all your mail offline, bad that you have to sync all mail twice (ie. All Mail + Label Folder). I hate this....you need to turn on Labs to support Advanced IMAP Controls, but even then there are tradeoffs involved. Gmail is a great e-mail system but far from a solid IMAP system. Their webmail system is stellar and far more advanced than me.com, however, MM's design "philosophy" is not tailored towards advanced users but average joes. Most people don't know what "server-side rules" are, so they don't see a need for them. As far as junk mail (spam), Gmail and MM are pretty even for spam protection--I've not gotten any spam in either Gmail or MM accounts, at all.
Files will sync just fine on iDisk via "iDisk Sync". I use it, my wife uses it, and it works fine. Some people complain of slowness during peak times but I have yet to see it either at home (20Mbps/2Mbps Cable) or on my university campus. Your speed is truly limited by the bandwidth of your connection to the 'net. If you have a high upload speed through your ISP, it'll work fine.
Some have mentioned Dropbox which is a great service for some people. I have tried but there are some things I do not like about it. Case in point, I e-mailed them about their uninstall method being incomplete. To their credit, they responded that same day and amended their FAQ (I even got an extra GB of storage space as a 'thank you'). Like I said, it's a great service and if you don't want/need any of the other services MM provides, then it's at least worth a try.
For me, I don't like to piecemeal things and like having all the services integrated. When I log into me.com, all my mail, contacts, calendars, idisk files, etc., are all there and I don't need to log into separate websites. $99/yr is not cheap but it's not super-expensive either, and you can usually find enrollment kits for as low as $60-$70 through Amazon.com .
