I'm simply gonna nod in agreement with cgaliher. If any of my customers wanted to make this move, I would make the same arguments as above
You have two points of failure when you move to a cloud solution (even MS's). 1 is your ISP and it's reliability. I've been very lucky with my cable business internet connection. Extremely rare for me to have an outage, but at least I have local folks to work and I have some control 2 If google apps go down (and that's happened more than my internet connection this year) I'd be lucky to get ahold of anyone who can actually give me any answers.
About 12 years worth of email. Find out how big your biggest mail box is and then see if Gmail has mailbox size limits
I think you'd spend hours trying to sync Exchange/Outlook/Gmail