It's an interesting book which summed up a lot of my experience working in startups. It's important to mention that the book focuses on testing new ideas rather than improving existing business.
Alright, so you have a new business idea and nobody has done it before, what's next? In short accept that you are wrong and your ideas are crap and proceed from there. Such mindset forces you to focus on feedback. Even if you found great market fit with 80% accuracy you still need feedback to make the adjustments.
The whole process should be focused on getting the market feedback and learning from it. The challenge is to get it as fast as possible and as cheap as possible. The sooner you fail the faster you will learn and the faster you will move on to a better one.
As a professional software developer I've found such approach very hard, because as a professional I'm responsible of quality, which I had to ignore if it conflicts with the delivery speed.
Also I've found my peers who don't understand that principle of growing new ideas, set the wrong priorities on tasks. They focus on the tasks which have medium to low business value, which is still not acceptable as that distracts you from the low hanging fruits.
I've also liked the "vanity metrics". Metrics which has no real meaning but makes you feel good by creating an illusion. So many businesses keep doing it. So true.
Lean Startup book was written from the author’s experience and I can personally backup most of the things mentioned in the book with my personal experience. That's why I've found it very useful as I see it as an experience exchange.
I'd say Lean Startup is "must read" only if you are starting new business, otherwise you are better off reading books on improving efficiency or quality of your existing business.