I've personally had a lot of fun playing with Twilio. Hint, though: don't decide to pay until you're done using up your demo minutes because those go away when you have a balance.

Getting to pick my phone numbers, etc, and have someone else host the infrastructure is pretty cool. I'm just trying to figure out how to really leverage it at the cost level to make it a worthwhile toy. There are a number of good ideas, but they require a lot of marketing effort to become efficient.

Twilio will succeed simply because they've reduced it to being ridiculously easy. I mean, a complete non-coder could practically put together a working app based on Twilio, which I doubt is the case with any VoiceXML provider.

I personally wrote a phone-in support service, company extensions directory and voicemail-to-email service with Twilio in literally a couple hours. It now integrates seamlessly into a new support system I'm launching for one of my companies in the next month or so. Twilio's part was so easy it was an afterthought, but will really make my new service shine.

Couple that ease of use with their super affordable and transparent ($5/mo + $0.05/min for an 866#) pricing model, and you've got a winner. I can appreciate the need for standards, but sometimes they're simply trumped by easier and better technologies. Look at the progression from SOAP -> XML-RPC -> REST for example :)

PS. Had an interesting issue with call forwarding through my wireless carrier and Twilio's support has been top notch in helping track it down. Can't say the same for my wireless carrier though... ;)