HoM in my opinion doesn't get the credit it deserves, its an incredibly useful items if used correctly but since a lot of people don't use it to its full capacity, they end up thinking it's weak or useless. So hopefully this guide can shed some light on how to get the most out of HoM.
Firstly lets go over what HoM offers, for 2050 gold you get two things.
+30 Attack speed.
The Transmute ability.
Transmute: Kills a non-hero target for 190 gold and 2.5× experience. Cannot be used on Ancients.
100 seconds cooldown. No manacost.
Now a lot of people focus on just the ability to use Transmute but the 30 bonus attack speed is quite significant in itself. Especially on the heroes who get HoM who can make exceptional use of the attack speed because their attacks have extra modifiers that benefit from more AS early game than more damage.
But before I go into the benefits of attack speed on these heroes, Transmute is a very useful skills that has many applications.
1- Increased gold gain.
Easily the main reason people get Hand of Midas, for the extra gold it grants them. When Transmute is used it gives 190 gold every time, there is a little bit more to it though.
Firstly, the gold it gives is Reliable gold, for those who don't know what that is, reliable gold is gold you gain from kills on heroes and assists, its gold that cannot be lost when you die. This is very useful for carries obviously.
Secondly, because it grants 190 gold every time, using HoM is a little bit more complex as the game goes on, because using it on a creep that grants 20 gold means you get 170 profit, while using it on a creep that gives 90 gold means you only get 100 profit. This means using HoM on small creeps is always the best choice for gaining gold, however because of the way Midas works with EXP you may want to reconsider how you use it early on.
Lastly, Midas pays for itself very fast, it sells for 1025 gold ( thanks to patch 6.79 changes to Midas) so you're spending 1025 gold for it if you sell it before the game ends.
2- Increased Exp gain.
As I mentioned above, using Midas on small creeps nets you the highest gold gain you can get, however Midas multiples the Exp you gain from using Transmute by 2.5 times.
Why is this important? because early on you will see many creeps you can use Midas on, and you will have a choice of which one to Transmute and the best in the early game are the large ones because they give much higher Exp but the same amount of gold.
Here's an example of what I am trying to say.
These two creeps spawn at the same camp, they are the Satyr Banisher and the Satyr Tormentor.
The Banisher grants 41 Exp and an average of 16 gold.
The Tormentor on the other hand gives 155 Exp and an average of 105 gold.
Deciding which to transmute is a big part of using Midas, since early game using it on the Tormentor gives you a whooping 387 Exp while using it on the Banisher just gives 102 Exp, so you can really pull ahead in the early game by using Transmute on the biggest creeps you can find.
Later on when you have pulled ahead in levels you can Transmute the Banisher to make a lot more profit than if you used it on Tormentor.
If you are going to use Midas on lane creeps, then always go for the melee ones. The give 62 exp compared to the range creeps who only give 41.
One last thing, Midas is instant in killing creeps, so by using it on the large powerful creeps you can save a lot of time and HP instead of having your hero kill them with auto-attacks that will take a lot of his health to accomplish.
3. Midas can counter some enemies.
Using Midas can give you quite an advantage against certain hero skills, from the top of my head these are the ones I can think of right now.
Chen's Holy Persuasion skill is a lot weaker against Midas holders, as they can easily use Midas to kill any creep Chen has. The same rule applies to Enchantress.
Axe's Battle Hunger requires the afflicted hero to kill a creep, that is easily accomplished with one transmute.
Dark Seer players usually hang back and Ion Shell one of their own creeps to move ahead and get gold for them, that can be countered by a fast Midas as well.
EDIT: as of patch 6.79 Midas can no longer be used to counter Necronomicon or creeps dominated by Helm of the Dominator.
Heroes that should get Hand of Midas.
Now obviously the best heroes to get HoM on are carries who benefit greatly from the increased gold and Exp gain, but some carries just benefit more from it than others, the heroes who I consider HoM to be core on are:
Naix also gains a tremendous benefit from HoM, since he can jungle or lane but he needs to farm a little bit to be truly useful to his team, and Midas grants him good damage since Naix just need to land as many hits on the enemy and let his passive Feast do all the damage.
Lastly Spectre desperately needs Midas, perhaps more so than any other carry, being an incredible hard carry with no farming skills is very rough, and Midas really helps solve her gold problems, plus its much easier to rush a Midas early on than to rush a Radiance. Sadly the attack speed does not go into her illusions and spec deals most of her early level damage from Desolate anyways.
Edit: Illusions do not benefit from Midas Attack speed, credits to ZeLover on playdota for bringing this to my attention.
All these heroes mentioned need Exp a lot, and that's why Midas is so essential on them, getting to a fast 6 unlocks very powerful ultimates. The attack speed is also very useful since they all have passives that benefit from it.
Of course there are other heroes who can use Midas well, such as Furion and Doom, but not to the extent those guys can. Its still very useful for them though.
And that's all for this guide, hope you all learned something useful today and thank you for reading so far.