Yeah, I'm actually completely wrong. My apologies to the Buble team. For whatever reason, I thought I saw an announcement of Buble essentially saying it was an opinionated feature-freeze of Babel with most of the configuration stripped out, but it appears to be a totally new project.
That said, the two are very different tools. Buble appears to be essentially string manipulation (which Babel does to occasionally with babel-template) built on top of Acorn. It's much simpler but there are a lot of useful transformations you simply can't do with that approach.
Edit: After some further looking into the project, I have to say that it seems like it would nice & easy to use for simple transformations, like the aforementioned React.createClass -> class Foo extends React.Component. However, you have to keep in mind that code that is not spec-compliant will break in mysterious ways perhaps months or years into the future. It may not be worth it, despite the ease.
`yarn install babel-cli babel-preset-es2015; ./node_modules/.bin/babel --presets es2015 <sources> -d <out-dir>` is not particularly difficult.