Guetzli is really hamstrung by its resource usage. When it first hit the news I tried it out, and compressing a full quality JPEG from my phone could take 20-30 minutes on an i7.
It depends on the context. If I'm converting a whole library of photos I wouldn't use it. But I've got a big hero area JPEG that's loaded as one of the first resources - I'm happy to run this tool in the background for a day to make it 20% smaller.