Let me start this way, I absolutely love the new OWA in 2013. I am a user that is on a bunch of different devices. I have an iPhone 5, iPad 2, Surface RT, MacBook Pro Retina, iMac at home and a PC desktop at work.
For my mail I use EAS on the iOS devices, I have beta tested some new mail replacements but they aren't very reliable, I will review them when they are released. On the Surface I use OWA, the built in Mail client really bugs me, I don't know why but I hate using it so I found OWA to work great. At work its Outlook 2013 and on the macs its OWA, I have been using Fluid to keep OWA in its own icon on my dock so I can always find my mail and not dig through tabs.
I like the Lync integration in OWA, I hope that Microsoft works to make it fully featured in future CUs becuase its a little awkward on the Surface in full screen IE; a new IM opens in a new window and it is annoying to switch back and fourth, it would be nice if it was embedded in the same window. Also starting a new IM isn't intuitive, searching for the user and then clicking the IM button is a little backwards.
The downside of just using OWA is notifications, yes the new mail sound does play but I don't always have my speakers on. My work around is having my phone on the table next to the device I am using (Mac or Surface) and when I see the notification of new mail I switch to OWA to read and reply to it or when the Lync mobile app shows a new IM I switch to reply. This problem isn't the fault of OWA, just of browsers in general. I had a user come to me two weeks ago and say that they loved OWA on their phone but didn't like how it was slow to refresh between looking at messages, again its not the fault of OWA but of the browser that needs to reload between pages.
Upgrading Exchange is also a social experiment, there are some remote users that I don't hear from until the Exchange upgrade who send me long emails on why they hate the new version because some of the navigation moved from the left to the top and now its blue instead of orange and emails from others who love the change.
The most interesting thing is that many Mac users have said how much they like the new interface, which is ironic.
Overall OWA in 2013 CU1 is a great upgrade over 2010. As the only user visible side to Exchange its important for Microsoft to keep improving it, I hope they add more features in future CUs and not make us wait until the next full version. I think that the new OWA is worth the migration to Exchange 2013 and that the other features on the backend are just a bonus for the admins.
I know some Exchange admins don't like the Exchange Admin Center, I have to admit I have only been in it a few times I mostly use Powershell for all of the administration.
I welcome your thoughts, what do you think about the new OWA?