Easy to use, great UI, flexibility, has been expanding based on listening to user needs over the last few years.
No good website integration.
I've been on board with Square for years, using it on my phone when I meet with customers. I much prefer it over the clunky POS systems usually in place. I love the way you can use it as a register in a storefront, but take it on the road if need be. I wish there was better website integration, so I could use it to do online sales.