Well, I can't speak for myself. I got 3 for free.
But although they are expensive, I feel like you should get one because it allows you to brag to all of your friends it will help give you experience for a comp. Same hardware. A GAN timer may be cheaper, but is it used at comp? It's like going into a simulation versus the real thing. A GAN timer may help you simulate a stackmat, but you will be able to have easier use with the stackmat, as it is very easy to use. No one will know that the reset button is the GAN button unless you read the directions. As for a stackmat, the buttons mean literally what they do.