A couple of months ago, I broke out of using Outlook on my desktop and killed off MS Exchange and I've been loving the results. I use a combo of GMail on the backend, Thunderbird+Lightning+Provider on my desktop, Plaxo (including Win Mobile software) for syncing contacts and GooSync for syncing calendars to my phone. It's all been working pretty well, except that GMail IMAP was losing message bodies left and right on my phone. It was a posted bug and now they seem to have fixed it. So, that leaves the fact that Lightning doesn't do offline calandering as the only buggy part of my experience. I didn't notice it for a while, because I'm pretty rarely offline, but its a real issue. It's also a slight performance issue, as it's always pulling calendars live... should def be caching for that reason if nothing else. If you switch windows, unplug, and then go back to Lightning, your calander will appear empty.
I'm sure that will get done soon enough, but all in all I've been really happy with my experience and have just cancelled my Exchange host.