I've been using both Gmail and MobileMe's mail for quite some time. I can say with absolute certainty that MobileMe has been much more stable and consistent than Gmail (not to mention the latter is scanned by Google for advertising revenue). Aside from a few hiccups when it FIRST launched, MobileMe has been rock solid. Conversely, Gmail has had a good number of outages, and this latest problem with accounts being locked out is just another reason to be wary.
I use Gmail as my junk/non-critical e-mail service and MobileMe as my important personal e-mail account. I may take issue with the price of MobileMe (though one would be ignorant to pay the full $100/year for it), but that is likely to change very soon (if we are to believe the rumors).
I'll take MobileMe over Gmail any day, free or otherwise.