After shopping around many SMS providers, I decided to go with The SMS Works because of their transparency (you can see exactly how much the service costs before signing up, including bulk discounts, and they don't charge for failed deliveries), and their modern and easy-to-use website and documentation (an afterthought for many SMS providers). The setup was simple and I was up and running within minutes. Purchasing more credits is easy and low credit warnings are useful so you don't run out. Being able to see delivery reports within the website is helpful for testing and monitoring purposes. I made the right choice!