We send some internal emails through Marketo to our sales team. It's a drip nurture of how-to documents and bits of information to make their lives easier without overwhelming with information. The hard part is creating employee records in and making sure those stay up to date. If you don't get too complex with what you are sending it's fairly simple to do. I have a suppression list that I use in 99% of my smart lists and that's how I keep them out of the prospect emails. A long winded answer, but that's how we use Marketo internally. Keep it simple and you should be fine.
I've added Employees to Marketo with a Status of Employee and then send out internal communications using that a Smart list to track success. However, like Chris mentioned, keep the data up to date with all the employees can becoming a pain. I've also seen employees added to the CRM as a contact as soon as they're employed and use that information to push through to Marketo.
Hope this helps.