I'll be diving into Beta 1 in a few days, waiting to see what others say and I'm bookmarking this thread - but I'm a bit confused by some of the comments here. I do use Office 365 and have an Exchange Server (2013) running, and I moved my company away from Google Apps recently since IMO I'm tired of the constantly-changing synching protocols.
About "push", MS moved away from push to streaming notifications in Server 2010 - EAS isn't real "push" either. I've configured Apple Mail/Calendar/Contacts for OS 10.9 and each works well (and I'm not even an IT rep) - notifications show up on our desktop and mobile devices within a few seconds of each other, and I'm looking forward to testing 10.10.
I'm a bit puzzled by the comments about Google's services relative to Exchange. Google doesn't use Exchange - rather it's "Google Apps Sync" for desktops and their own sync service for mobile devices (they use/used their own modified version of EAS for mobile and never published their protocols). That they changed their sync services several times since 2010 led me to pull the plug on Google and look elsewhere. Given that MS charges per CAL, I expect to pay for their notification services as it's performed better than any XMPP-based notification client I've tried.
As for comments about Server 2013 and Office 365, the notification subscriptions, mailbox events, and EWS in Exchange are different than other versions of Exchange. I'm cutting Apple some slack in this regard - OS X 10.10 is a beta. I've configured my email clients for the EWS requirements, and I have no problems whatsoever with Apple's clients or Office 2011/2013. I hope that Apple gets their future betas working well...