Why not just use Reserved Instances?

Reserved instances are a great way to save money on your AWS bill if you can predict your instance type usage levels reasonably well into the future. CloudOptimo can still help you save more though, since spot prices are typically significantly below the reserved instance price for the same instance type, and do not require a significant upfront commitment like reserved instances do. They are two independent strategies that can help you reduce costs, and you should apply both if you can. As far as working together, CloudOptimo will work seamlessly with your reserved instances, using them instead of on-demand instances when they are available.