Is it worth getting in a premade match? Is it overpowered in any sense?
I agree with Nobody's answer with the article link highlighting it and that it is an item that seems to have been built for Tryndamere and given his ultimate is a tad overpowered for him. The shield only absorbs magic damage but it will not count as health and recall Trydamere's passive increases his crit chance by amount of fury. As well as the fact that it gives bonus attack damage that powers up his spinning slash damage, so this item is just a monster for him. Another champion that many overlook who could benefit from it (though they are touched upon in the article I am happy to see) is Olaf. Any champion who plays with a low health bar constantly could benefit from this item.
I don't think it's that great an item, but if you want to read more there's a well put together article here: http://clgaming.net/forums/board39-reign-of-gaming/board40-blogs/board82-a-different-view/5950-guest-post-hexdrinker-and-the-maw-of-malmortius/
I think the champ it works best on is probably Tryndamere because of his ultimate.
Definitely check out the in-depth analysis linked by Nobody.
As with most items, its a very situational purchase.
I personally go Hexdrinker > Maw of Malmortius only on melee AD bruisers when I'm facing an AP heavy team. If you faced off against an AP caster during the lane phase, it can really help your effectiveness. During teamfights, the passive MR helps mitigate AP area effect damage, the AD helps kill your target faster, and the shield helps when you're getting focused. Couple a Maw of Malmortius with a health item to prevent burst from killing you, and the Maw's passive shines.
If the champion is more attack speed oriented (Xin), I'll rush a Wit's End first, then follow it up with the Hexdrinker.
Maw of Malmortis is NOT a must buy item for anyone. This item is more of a niche pick. There are only 2 times you should consider getting this item.
1) You are laning vs a tough AP top and you are an AD based champion who needs additional MR to win lane. (Ex. Poppy/Jax/Renekton vs Vladimir/Ryze Top)
2) You are some sort of carry or bruiser (Jax/Irelia/Renekton Top, or typical ADC) and the enemy either has multiple strong AP threats or a single fed AP threat and you don't want to hurt your damage output too much by getting a defensive item.
Even in these situations, often you will only need to purchase a hexdrinker as it gives you the majority of the defensive stats and the defensive Magic bubble.
TL;DR; This item should never be part of your core build for any champion. It just doesn't have strong synergy with anyone in particular and is only used as a counter to magic damage when you are trying to not sacrifice too much damage.
but with Xin if you rush wit's end first u get the attack speed but not the damage output. Try going phage first its passive gives you some utility too and health and damage. then go for wit's end or hex drinker this will work if you start your lane phase with a doran's sheild.
also another option..(no idea if it will work in all situations)
spirit visage can also add your self heal with the magic resist you need with some health bonus may be if you don't focus on the damage part in the beginning and be tanky you can withstand and damage from a ability power source...
suggestions are welcomed.