I'd stay away from Office 365. The Web Apps are perfectly fine, but being stuck with SkyDrive Pro (or whatever they're calling it now) or SharePoint Online to manage your shared docs is not particularly pleasant. If you also moved your mail to it, that might start to change the tune a little bit.

I also have to caution you... before moving to a cloud solution, make sure it is in fact up to serving the needs of the business. Usability is one thing... users certainly demand usability these days... but other needs are often overlooked, such as access control. In the past, I've seen teams go rogue and decide they didn't like the company's SharePoint or file servers and were going to start using something like Dropbox on their own... then went running to IT six months later when the team leader was terminated and they were trying to figure out how to keep all those team documents in the folders he owned accessible to his team but revoke his access. As you move to the cloud, the most important thing is to ensure you're retaining the amount of control that you need to have over your own (and potentially your customer) data.