It is not that good, but it is pretty good. Particularly it can help you in lane matchups against magic damage lane opponents if you are playing something like
and you pick up hexdrinker to help you in lane. The item seems to be designed to help bruisers all in an opponent without having to worry so much about getting bursted, even gaining power for losing hp.
There are some champions who synergize really well with the item:
all do well at lower health, so maw is more appealing. But as it sits, most champions will just sit on hexdrinker because it helps them in lane and they only finish maw as one of their last items. On ADC's and Assassins (champions who go full AD), Mercury Scimitar is usually preferred as it allows them to escape from getting CC'd out of a fight, which is more important. In addition, ADCs should almost never buy Maw (
is allowed to because he is truly a bruiser) as it implies that they are going to be low health to make the most out of the item, which ADC's (ideally) should stay untouched for the majority of the fight.
I think Maw is a good enough item right now, but it is rather expensive, so it is unappealing to a lot of champions. It's a good item situationally, but I wouldn't build it EVERY game, because it is not always the best option, even against a high damaging AP stacking opponent. For instance; when you are facing Annie, you will build Banshee's or QSS as your MR option because her stun will be negated, unless your item is on CD. In lane you might pick up Merc Treads, because they reduce the CC. You CAN go hexdrinker when laning against an Annie, but if you focus on building too many MR items, you will either be lacking damage, or not have enough physical defenses for other members of the enemy team.
Those are my thoughts on it. Hexdrinker into Maw I find is very situational, but it's not a bad item. It really helps a lot of Toplaners against Rumble for instance, but again, I don't think it is good for every game as it is expensive and implies you are going to die or be near death a lot to make use of the passive.