Do people actually use Amazon EC2 Spot?

Whether or not to use spot instances would depend on the application you are using.
For example, it may be used to reduce costs in applications where there is no problem even if the process stops halfway due to capacity limitations on the AWS side.

Spot instances are suitable for cases where a large amount of data is processed in a short period of time, for example, in batch processing. However, if the end time is critical, it may not be possible to make it in time if the spot instance is stopped, so it is advisable to design the system with a sufficient margin of time.

On-demand instances are expensive and reserved instances are not applicable when a large number of servers are needed in a short period of time, such as for a game application testing environment. In such cases, preparing the environment with spot instances can significantly reduce costs. However, it is a good idea to make sure that the members executing the test fully understand that the environment is configured with Spot instances.

In addition, in conjunction with these methods, SpotFleet and AutoScaling groups can be utilized together to make the use of spot instances more friendly.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-spot-instances.html

A Spot Instance is an instance that uses spare EC2 capacity that is available for less than the On-Demand price. Because Spot Instances enable you to request unused EC2 instances at steep discounts, you can lower your Amazon EC2 costs significantly. The hourly price for a Spot Instance is called a Spot price. The Spot price of each instance type in each Availability Zone is set by Amazon EC2, and is adjusted gradually based on the long-term supply of and demand for Spot Instances. Your Spot Instance runs whenever capacity is available.

Here is a workshop that walks through services and scenarios for using Spot, AMAZON EC2 SPOT INSTANCES WORKSHOPS.