However, they can be great to help fill out your server cluster with things that don't always need to stay up (like Mongel servers). Buy some reserved instances, then put out several spot bids as well for things you can load balance to.
That said, keep in mind that if you need to keep at least one instance up, you need to put them in multiple availability zones, or vary the max spot prices - if there's a load spike that leads to your spot instances being terminated, it'll likely hit all of the spot instances at a given price in the same AZ.
