Jhanic states mostly. Proper meditation gave me a new perspective on Zen and Buddhism as a whole. I've studied it extensively before and disregarded meditation as being superfluous. I was really wrong. It helped tremendously with understanding esoteric parts of Buddhism.
The first experience that got me 'hooked' or rather made me realize that it's not hogwash, is the arousal of piti. The warm sensation in solar plexus that came out of no where after long breath meditation concentration and it turned into waves with each breath-in. It's hard to describe but it's unmistakable when you start getting better at meditation. That book covers everything.
Make no mistake, it's not easy. For whatever reason I got lucky with being able to tune in to deeper meditation, some people struggle for weeks and months to get there. I did Zazen previously on and off and this never got me anywhere. Anapanasati meditation is what will get you to jhanas. From then on, as some scriptures say, the real path to Buddhism begins.
Even if you are not into Buddhism, the pay off is still tremendous. Here is the practical and measurable result of proper deep meditation: ability to calm your mind and achieve total equanimity with virtually no thoughts, just pure awareness and tranquility that lets you sit for hours if you wish to, with zero desire to even move an inch, just sitting still. The ability to exercise your brain like that is pretty huge. In the long term and consistent practice it's beneficial to overall well being.