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google doesn't do back to my mac and doesn't sync anything but contacts and calendar. MM syncs bookmarks, dock items, and more. It also allows scheduled backup to your iDisk. Oh, and doesn't mine your data to sell to god knows who.
I would love to de-google myself, but I can't leave gmail because MM mail isn't good enough (mostly it's the performance). If they can make mm as fast and reliable as gmail, I'll gladly ditch gmail.
Other than that (and iDisk being slow), I love mm and think it's absolutely worth the money.
Just a few points regarding MM vs. Google Sync:
As far as bookmarks go, MM does sync from Safari on a PC or Mac to Safari on an iDevice, but it doesn't provide any web based access to them. You have to edit your bookmarks on a device that you have set up for sync while Google's Chrome based sync integrates your bookmarks into your Google Docs for easy editing and sharing even when away from one of your devices. Plus, the newest version of Chrome supports preference sync, too.
The system-wide preference and Dock sync that MobileMe provides is nice in principle, but even though I use three Macs, I've yet to find a great use for it. I find that I want my preferences and Docks to be different on each computer.
While not integrated with the MobileMe or Google suites, LogMeIn Free is far better and more reliable than Back To My Mac (try it,
www.logmein.com). I've actually phased out my use of Back To My Mac entirely in favor of LMI, Apple's VNC implementation is inadequate at best.
Using Backup.app to send files to iDisk is unbearably slow and has very limited options. For backing up your entire Home volume or specific types of files, Mozy is quicker and less intrusive. Plus, Time Machine backups are much more robust and there are numerous ways to upload your Time Machine backups to online storage (Time Warp is one). You can also upload any file under 250MB to Google Docs.
On balance, I don't think MM's service is worth the cost due to the vestigial nature of its web services and incomplete software features even though it is tightly integrated into OS X.
[I just saw your followup and agree that there is a subtle cost associated with Google's services if you're against having your private data aggregated and sold. Remember, there is Google Apps Premier for $50 a year per user that, at least, gets rid of ads in Gmail.]