Another happy Harp user here. The team behind Harp has also created Surge (http://surge.sh/) which is simple & well thought out.

I find that Harp and Surge get in the way the least, thanks to "convention over configuration". Just give me my Jade & Stylus, thank you very much.

I'd be interested in hearing how others use Harp with current tooling - React / CSS modules / JSPM anyone?