[bmelton]

As dagw said, it pretty much depends on what you're doing.

We have dev, test and stage instances that are almost always off. I'm not going to reserve those at all, because I won't generate enough charge on them over the course of a year to even meet the initial payment.

For our production instances, yes, obviously we're reserving, but there are lots of cases where you wouldn't. Even with production instances, we occasionally spool up another instance to handle load spikes, for which we don't boot a reserved instance.

In another of our apps, we actually spool up a new AWS instance to handle customer requests. The user clicks a button on a website, uploads a (large) file, which we spool up an instance to process for awhile, return the results then shut the instance down. That kind of usage isn't really suitable for reservation.