Yep, we're (heavy) users of Twilio SMS. Aside from maybe two incidents that I can think of in the last 3 years, they've been rock solid for us. I am struggling to remember any customer complaints from SMSes not being delivered -- while I think our scale guarantees that this has probably happened (yay telephony), it doesn't happen often enough for it to impact customer results for us. (The typical customer will be sending dozens to hundreds of them a week, so one or two going undeliverable do to carrier issues/user error/etc is not the worst thing in the world for us.)