I read only what I needed at a given time but since its release I've probably read the whole thing, just not cover to cover. (Sans the Magic Item descriptions)
I like the world building stuff, though if you are experienced in this or already have a world it isn't that useful. Adventure creation seemed. . . fine, though if I create my own adventures I hardly use it. (I use the Lazy DM method from Sly Flourish.) Encounter building is garbage. Way too complex and the balance doesn't work anyway. Treasure tables are good.
Optional rules are. . . meh, hit or miss. I've never liked any of the optional rules about damage or healing as they seem to mess up the game. We used to use Massive Damage for a little while but it seemed OP and so it fell out of favor, getting used rarely. Both of my groups use flanking and when I DM, I use all the optional combat moves like Shoves/Tumbles/Overrun though most of my players seem to forget they exist and frankly aside from a few niche situations (a chokepoint or a sharp drop) they don't seem useful.
Overall I don't care for the DMG too much. Its my least used book and is only frequently referenced for Magic Items.