If you want to talk about guitars and ambience, you must mention
Labradford. I posted a review not long ago. One guy plays guitar,
another a motley assortment of moogs. Most of their recording is done
in a big echoey hallway in the basement of a university building, so
they echo is natural rather than digital.
They have some songs with words, and no beats, so it's not exactly IDM, but
it is pretty ambient stuff.