The built-in Less compiler works fairly well, but has a couple issues that were

annoying me.
First of all, in order to cope with an old bug in lessc that no longer exists,

it redirected the output of lessc rather than passed the output filename on the

command line. This made it impossible to use the --source-map argument, which

makes devtools a much nicer experience. I've reimplemented compile_file to

pass in the outfile name on the command-line, and raised the question with the

django-pipeline maintainer to see if we can't get that upstream.
Second, the compiler was completely ignoring the outdated check and always

recompiling every file on every load. This made sense because there was no

dependency analysis, so if an @imported file was changed, the dependent file

would not be recompiled. This was slowing down reloads a fair bit (but it was

still faster than recompiling in the browser).
To fix this second part, I've written a helper, less-imports.js, which uses

the less module's parser to get a deep list of the file imports. These are then

each compared against the target output file's timestamp to see if anything is

out of date. While this is still some overhead (since it still has to parse

the files), it's quite a bit faster than actually recompiling.

Loaded and reloaded things a bunch.
Made a change to defs.less and saw in the log that it was recompiling all the

  files that imported it.