What does the software or service do well? I like that it's super easy to setup, comes with a couple free themes that are nice looking, and they don't charge anything to use the basic, necessary features. It helped me get multiple shops running quickly and easily, which is a good thing for someone on a budget.
What does the software or service not do well? I dislike that they don't have more free themes because the paid ones are extremely pricey. Also, the calculated shipping feature they have built in doesn't always pull accurate shipping costs and won't let you customize it on other shipping methods. One thing I would definitely look into is the fact that you need to constant re-link your square account if you are using that as a payment gateway.
What do you wish you had known about the software or service before using? I just wish I had known about it earlier, overall. I used other software that was costly when I could have been using WooCommerce early on. I also wish I would have known earlier all the great free plugins that can be found to enhance woo-commerce. There really is no need to spend money when using it. If there is a plugin you wish existed, simply search the Wordpress plugin directly and surely someone has though of it, for free or a light version at least.
Have you used a competitor to the software or service? If so, how does it compare? I did try Shopify. While the interface was great and it made the design process very easy, I disliked how extremely expensive it was. If you have basic knowledge of Wordpress, have a good theme and can problem solve, then definitely go with Woo Commerce instead. It will save you a ton of money.