Even if you are a Rails shop first, you can use DDD to determine if it's wise to take on the project in the first place. If it turns out that the domain cannot be expressed as a CRUD app, maybe refer the client to someone else. Then you will officially have used Rails with DDD, even if you haven't written a single line of Rails code for the project.

Much of DDD is really tech independent, and takes place before a single line of code is written. This is probably also where it's the most important. You're going to have to learn the domain first or the project will be a world of pain regardless of what architecture you choose.