My current preference is the 'browserify-rails' gem. I set up a package.json file to manage my javascript, but I let the regular asset pipeline handle the css and images. I tried webpack for a while, but it seemed redundant when I already had the asset pipeline.

I also like to add 'babelify' to my package.json and 'config.browserify_rails.commandline_options = "-t babelify"' to my application.rb to get ES6 goodness.