Our service (frucall.com) heavily relies on SMS, and I had very bad experience with ClickATell. While their web site looks pretty legit, their service is very unreliable (and by the way they never sent us the refund after we canceled with them).

It is important to notice that 2-Way SMS in the US has strict regulations - you have to use a short code, otherwise your service is considered illegal and the carriers can shut you down. ClickATel does not operate based on shortcodes, they use SMS modems which is basically a way to hook up a cellular line to a computer and intercept SMS messages.

There's quite a bit of details that one should be aware of if one wants to run a business grade SMS service in the US due to the way carriers control the SMS traffic. If there's interest from the readers I'll be more than glad to write down some of the details.