SMS poll texting for live presentations?

anyone used this before in their presentation? I'm looking to use one for my upcoming class presentation with about 75 participants. I want them to be able to respond to few questions using sms (assuming they all have unlimited texting so they don't pay anything) and I'm the only one paying for the service. I also wanna be able to show the real time data coming from the poll.

I'll appreciate any advice/input on this

Thanks,