I prototyped something in a couple of days with Python and Flask and it made a good enough impression that we are thinking about fully developing the idea. I'm open to just about any tech and am thinking about continuing with Flask (and Flask-restful), using Go, or Ruby on Rails. I have almost no experience with any of these and that's part of the reason they appeal to me.
I'm going to be honest with you. I love building pure REST apis in rails. The experience is so clean and effortless.
That being said there is at least one caveat to using RoR for the first time, and thats that the learning curve is probably going to be a little steeper than a rest lib like flask where things are more explicit, since there is a lot of convention over configuration in RoR. Once you learn the conventions, the development speed is ridiculously fast, but until then things can be pretty opaque and take some time to get used to, especially for people who don't understand well how the ruby runtime works. I've witnessed this on multiple occasions with multiple projects, if there are more advanced/not-stack-overflowable things that need to be done, devs who are working in RoR and ruby for the first time, will run into a lot of roadblocks. This has little if anything to do with their particular skills as a programmer and more to do with the dynamic nature of ruby and a lot of very tricky hacks rails does under the surface to make the end-user api clean and succinct.
Even if it is a little excessive (which I don't think it is), I have features planned in the future where having an ORM like ActiveRecord+Ruby will make things very simple.
I'm a bit of a REST newbie and so some concepts (esp HATEOAS) are still a bit blurry. I would probably take a little time to dig into just what goes into a well-structured REST service.