It's difficult to say. I've worked at two companies that have used EC2 for various purposes. One had an instance that was used for dev work get "corrupted" on three separate occasions (I couldn't get the specifics, but there were definitely I/O issues with the instance storage), and the other has been using the same production box for more than a year.

The bottom line is that EC2 installations need to be designed as semi-permanent. My preferred strategy is similar to how Google talks about their hardware (when they do), that any one server can go down at any time, but the overall setup is resilient to failure.