I've been running Exchange Sync ever since the 3G, and occasionally have to delete the account and then set it up again to get rid of issues and speed things up. On the iPhone 4, I've already done this twice due to a locked up calendar (the calendar app will load, but then I'll get a spinning wheel in the middle with the words "loading..." that will never end).
Thankfully this procedure doesn't take that long.
I'm not sure if this one is Apple's fault or Microsoft's. Exchange is bloatware.