My company is an Exchange shop – amongst other things, we develop software that integrate with Exchange server for calendar synchronization purposes.
As we “eat our own dog food” – or “drink our own champagne” if you will – we run Exchange server. We are a small shop, so we only have one server, with no redundancy etc.
In case of server downtime, we need a backup MX hosted somewhere other than our datacenter, so incoming mail isn’t lost while the Exchange server is down.
We used to have a VM with a traditional hosting provider, but that setup became too complicated (the “best” way for us to pay them was with Paypal, and there were problems with them not being able to verify our VAT number after we were merged with our parent company).