I know I sound like a bitter old man here, so just for the record: the C7 does a lot of things really good (camera, SMS, actual phone calls, bluetooth PC connectivity, just to name a few). This is not a balanced review. This is a description of my problems during my 3 week long ordeal to get the C7 working properly in the mail/contacts/calendar area.
With that out of the way - rant incoming
Overall I'm not too impressed with either email, calendar or contacts in C7. But some of my problems may be due to pretty "businessy" requirements for a media-centric smartphone, so they may not apply to you.
All areas mostly work, but have some issues ranging from minor annoyances to (in my oppinion) critical omissions.
First of: you can only have one Mail for Exchange connection on the C7. This MfE connection can then be targeted to your Exchange account, or your Google account (possibly others as well, I don't know). But only 1 MfE connection total.CALENDAR
The "one MfE" restriction becomes a problem here, since the calendar can only sync to two sources: Mfe + OVI Calendar. So you cannot sync to BOTH your Exchange Calendar and your Google Calendar (*). In my oppinion OVI calendar is not a viable alternative to Google Calendar, since it can only be accessed in two ways: from calendar.ovi.com, or from your phone. This excludes Mozilla Thunder-/Sunbird, Windows desktop widgets, other phones etc etc. Before OVI Calendar is "opened up" through CalDAV, iCAL or similar API's its a boxed-in deadend you might as well just forget about.
The calendar app itself is no beauty, but functional. A major flaw is that you cannot accept or reject Exchange meeting invitations! Although thats propably an error in the email app, not the calendar app. Nokias Q1 2011 scheduled major update should fix this accept-meeting issue. Hold your breath at your own risk.EMAIL
Synchronization is poor, so poor. Push-email is only supported through your MfE connection, so you cannot have push-Exchange and push-GMail. Currently Gmail over MfE is said to have errors so you simply cannot send mails at all, and have to set up a seperate mail Gmail connection just for that. I haven't tried this out myself (since my MfE connection is dedicated to my work-Exchange server), so it may or may not have been fixed already.
MfE-sync to Exchange seems to be working as intended though. No issues found there, so score one for Nokia
Gmail can be synced via Nokias servers (default) which supports push-mail, but are horribly unstable which means you often get ½-a-day without any sync at all, and get incorrect sync even when the servers are up.
You can also sync Gmail directly to Googles IMAP servers, but then you lose push-mail support and have to settle for old-school "sync every 30 minutes". Direct IMAP to Gmail seems to be working on all other areas though.CONTACTS
Overall its very good. Contact information is well organized, and the option to display a standard-number for calls/sms/mails etc on the contact "front page" is nice.
Integration with social networks is a joke. There's a big "Social Networks" button you can click, which (most of the time) will open up the Facebook/Twitter app and display a friend if one is found with a 100% name match. From the facebook app you can then copy an ultralow-resolution profile picture to your local Nokia phonebook, and thats it. They might as well not have bothered.
But thats a minor issue, overall Contacts is great. Except for synchronization, which gets a "just passed" :-/.
You can sync with your MfE connection, with local PC Outlook/Windows contacts, with OVI Suite, and with contacts.ovi.com. All of your synced sources will be merged (!), so you cannot f.ex maintain a list of business contacts in Exchange at work, and personal contacts in OVI Suite at home. You'll end up with one big list of everything, everywhere.
With that said, sync works most of the time. I've had a few cases of lost/misplaced data, but that appears to be mainly when syncing with contacts.ovi.com. That site looks unfinished anyway so unless you really need web-contacts, just edit on the phone / OVI Suite, where it works (when OVI Suite works, that is).
Overall, I'd not recommend the C7 if email/calendar is important to you. The Q1 2011 update might improve the situation, but then again it might not.
(*) Unless you pony up the 20 euro for Googasync or similar.