I don't have many end users (using FreeSWITCH as our office PBX), but I've just started using Flowroute's SMS API, and it works well.
I have possible future plans, but at the moment I have a PHP script that takes any SMS message received on any of our DID's and turns it into an email. Obviously the volume is low at the moment, so I just forward the message to the recipient, but eventually I want to build an email to SMS gateway.
Read my earlier message on this list for how I am using this SMS API to send a text message when a voice mail is received, using a LUA script, mod_curl, and the Flowroute SMS API.
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Steve