There are many products meant to help you migrate to Office 365. The following are my three favorites:
In my usage, CodeTwo Office 365 Migration has been an easy way to get your on-premises. Exchange mailboxes into Office 365's Exchange, whether you are doing a complete move of all users or moving some to Office 365 and keeping some local in a hybrid approach. CodeTwo helps move mailboxes and public folders from on-premises Exchange 2003, 2007, 2010, and 2013. It can also move mailboxes from Google Apps for Business and individual Gmail accounts to Office 365. The best part about this tool is that you can get it for free: Enterprise Office 365 subscribers can get a migration license paid by Microsoft if the enterprise designates CodeTwo its Office 365 migration partner of record.
Mimecast Unified Email Management helps with on-premises, hosted, and Office 365 deployments, depending on the components you license. They nicely fill gaps around enterprise-grade archiving, antispam/antimalware, continuity, and large file sends. For example, the archiving feature ensures that users can access their sent and received email and restore it if necessary themselves, saving you time and hassle as an IT admin. But they can't delete archived emails, so your organization's e-discovery and compliance remain intact.In my experience, Mimecast's continuity feature -- essentially a cloud-based failover server for Office 365's Exchange -- is very useful. Office 365 goes down at times, as my company can attest. Mimecast keeps your email up and accessible during those outages, so users aren't affected. When Office 365 is back up, Mimecast brings Office 365 back up to speed.
With more and more services, infrastructure, and platforms moving to the cloud, the need for good third-party tools will only grow to enhance the core offering, such as Exchange. Some of those tools exist today, and I encourage you to use them.