Does a variable-speed pump really save money?

Yes, it will pay for itself in electricity savings within a year or two, depending on your rates and run time. Single-speed pumps waste power by running full blast constantly, while a variable-speed pump lets you run at low speed for normal circulation (using a fraction of the power) and crank it up only when needed. You also get less noise, less wear, better water clarity, and most models qualify for utility rebates.