How do you measure the success of your products, and what metrics do you use to track progress?

It highly depends on which stage your product or company is currently at. For a startup, mainly focus on customer related metrics, like # of customers, # of new customers, customer churn, to ensure that you're developing a product customers want; For a non-startup, it's much more complicated and should focus more on business related metrcis, like MRR, ACV, unit econoics, to ensure that you're running a success business based on this product.