Our current proposed structure would be to use AWS for front end and Azure for the DB which isn't ideal but I find AWS far more flexible for our purposes.
A windows 2008 server license on 2 machines for fall back comes in at around £700 - £800 a month per instance. The same redundancy on Azure is around £80 - £100 a month. If it doesn't work then we go back to the drawing board, if it does work then we have saved ourselves an almighty chunk of money.
We cache a lot of data in our application too which we are putting into elasticache, which is local to EC2.
We haven't used it in great stress yet but initial tests were very good
If azure opened it's model up then we wouldn't need to do all this!
Though they're probably still ephemeral, but then again, so are EC2's...