I've been looking at various options for student response systems - primarily clickers - and have been quite frustrated at how that market is set up. I wouldn't be able to recommend use of those, in good conscience, given the recurring costs. It also seems a little strange to compel students to buy a specific piece of hardware to perform the task, when they (almost) all have laptops and/or smartphones in their hands anyway.
Why not just use a browser-based polling system to solicit and collect student responses during class? Ideally, it should be easy for the prof/instructor/TA/teacher to add new questions on the fly, which appears to be a weakness in the current web-based polling systems. But it feels like a much more productive approach, rather than sinking money into vendor-locked hardware that students have to pay to unlock each semester.
This isn't the first time I've looked at web based systems to handle student responses and feedback. Last summer I was looking for something to fit the bill, and didn't find anything that really did what is needed - a simple, frictionless, flexible app that lets profs (or anyone else) ask a question (or a series of questions) and gather responses in realtime, and also allow on-the-fly modification and posting of new questions.