The Exchange Team made a number of interesting announcements at the Microsoft Ignite conference, including support for Exchange Server 2013 servers on Microsoft Azure IaaS. Previously Microsoft supported placing a DAG witness file share in Microsoft Azure IaaS, but now full Exchange 2013 servers are supported, even for production workloads. One thing to note is that the Exchange Team is requiring Azure Premium Storage for any drive on the Exchange 2013 server that stores a JET database. This includes the transport database, so if you install Exchange to the C drive you need to use Premium Storage for C or you need to move the transport database to another drive.
There are a number of scenarios where it may make sense to use Azure Iaas rather than Exchange Online or an on-premises deployment. The main example given was the placement of a Hybrid server hosting no mailboxes. In many environments one Exchange server is left standing after an Office 365 migration for mail relay and AD object management purposes. Since this type of server requires little storage it is a relatively affordable role to place in Azure IaaS. Microsoft still recommends Exchange Online over Azure IaaS if you are looking to move your mailboxes to the cloud, but they will now support you if you place mailbox workloads in Azure IaaS.