In my opinion SMS two-way messaging solutions suck. GSM modem solutions involving a phone attached to a server gateway are not guaranteed to be reliable (reception, telco's don't aprove, etc.) and have a hard-to-remember number to send to. Short codes are extremely expensive. Existing shared short code solutions have restrictive or clumsy API's and are still more expensive than they need to be.
What do you guys think? What bothers you the most as a developer looking to incorporate an SMS messaging component into their application?