With the new rework Heimerdinger as a champion has become much more viable. His turret deal much more damage than before, his w much more reliable and his e is easier to hit. His ulti has been changed significantly and I will talk about that later.
In this guide I will talk about the best way to use his abilities and talk about how to play him in the early mid and late game. It will also give you an idea of what matchups you should play Heimerdinger in.

Pros

great farming/pushing ability

reliable damage with w

many and varied forms of damage

surprising burst

Cons

has a hard time vs long range mids eg. Lux

falls off slightly late game in terms of damage and survivability

no escape at all

prone to ganks because of this

The masteries in the offensive tree are quite straight forward with all the AP masteries being chosen. In the utility tree I choose the mana regen and max mana to help me spam my w for more poke. I have also chosen to master both summoner spells to help me escape more often(flash) and deal more damage (ignite/ exhaust). There are also some specific masteries I would like to talk about

Sorcery

This mastery I great for Heimerdinger because not only does it reduce the cooldown of his abilities but it also reduces the time it takes to get another turret kit and makes his turrets fire their beam more often. I do however only have 3 points in it because I think mastering smite/ exhaust is more important and you need 9 in the utility tree to get increased buff duration

Meditation + expanded mind

These 2 have both been maxed out in the utility tree because they provide great lane sustain with mana and mana regen before you have a highly stacked tear of the goddess and a chalice of harmony

Runic affinity

This mastery I great for Heimerdinger for similar reasons to meditation and expanded mind in that it offers much more mana sustain. As Heimerdinger you should always be trying to get your blue buff in order to poke and farm more effectively this is very useful because it allows you to bully your opponent out of lane and sets you up with lots of gold for the late game

My marks are quite straight forward with flat magic penetration to maximise damage output and poke potential.

I find armour seals to be most useful because they provide good protection in the mid to late game when ad carries are the most important people in teamfights and they also help against ad mid laners which are on the increase.

For my glyphs I take 5 flat ability power and 4 flat magic resist. This is because the ability power glyphs help with early game poke and damage and magic resist provide a bonus to survivability. I don't find it necessary to have all magic resist glyphs because I get a chalice of harmony early on and that gives a good amount of magic resistance for early game.

I go for flat ability quints because it grants great early game damage and allows for even more op poke and early game burst with his ult and w. I prefer these over scaling ability power quints because I find that my build deals good late game damage anyway and also Heimerdinger can really dominate the lane phase with these.

flash

ignite

exhaust

barrier

Flash is great because it provides an escape for a champion which has none. You can often stun or slow an enemy with your e before flashing away to safety. this enables you to dodge ganks or to get out if things get hairy.
Ignite is a great offensive summoner spell because it allows you to finish off fleeing opponents or to deal more damage. it also has the benefit of countering some mid laners such as swain that rely heavily on spell vamp to stay alive in fights
Exhaust is a useful alternative to ignite in place of an offensive summoner spell. The reason this works for Heimerdinger is because a lot of his dps comes from turrets and this summoner spell allows him to keep the enemies in that area for as long as possible, it also has the benefit of reducing their armour and magic resistance if mastered
Barrier can be useful as an alternative to flash as a defensive summoner spell vs assassin mid with low or no cooldown gap closers such as Akali or Diana. This is because even after a flash they will often be able to jump straight back towards you thus rendering your flash useless.

Techmaturgical repair bots:
Many people often forget of ignore Heimer's passive but this is just plain WRONG. Even after the nerf it still allows you to heal up your own turrets to keep them alive but it also heals your allies to keep them alive. Furthermore because it heals you then it allows you to trade with most enemies in mid lane and still come out on top because of the boosted HP regen it grants

H-28G Evolution Turrets:
These little guys have received some serious damage buff in the new rework. Not only do they have increased range but they also now fire out a large laser blast every 12 seconds (effected by cooldown reduction). Probably the most important part of the new update is that Heimer can now have 3 turrets(again). They do also have lower HP than before so they can be killed much quicker.

Hextech-micro Rockets:
With the new changes to this ability not only has it become more accurate and reliable but it now has many and varied ways to use it. You can have them all converge on a single target by directing your cursor right at an enemy champion or you can have them more spread out to have more AOE damage or to poke down larger groups on enemies(this can be achieved by moving your cursor closer to you own model).

CH-1 Electron Storm Grenade:
This is now a much more standard ability then before because it now travels much faster. It can now be hit much more often and much more easily. It has the ability to reliable CC multiple targets and if Heimer only really innate CC or escape(you can stun someone and run away)(if that even counts)

Upgrade!!!:
This has received the largest change out of all of Heimers abilities and can now be used in many different ways. In this I am going to talk about buffing each ability separately.

H-28Q Apex Turret:
Buffing Heimer's Q will spawn a H-28Q Apex Turret for 8 seconds. This turret deals MUCH more damage than a basic turret and is by far the best for outputting constant DPS in a teamfight. It isn't as useful in a 1v1 fight in mid as Hextech Rocket Swarm though because Hextech Rocket Swarm deals far more burst damage.

Hextech Rocket Swarm:
Buffing Heimer's W will send out a Hextech Rocket Swarm which fires out 4 waves of rockets that converge on your cursor in the same way to your W. This alone would be significant burst from 1 ability but each individual rocket also has significantly boosted damage compared to a rocket from a W. This is the best ability for bursting down key targets in fights or in a 1vs1 to kill you opponent quickly or scare them off.

CH-3X Lightning Grenade:
Buffing Heimer's E will cause him to toss out a CH-3X Lightning Grenade. This grenade only does slightly more damage than his E but it has a much larger radius than a CH-1 Electron Strom Grenade. It also bounces 3 times stunning, slowing and damaging nearby enemies with each bounce. This is the best CC ability in Heimer's kit and it also the best for killing fleeing targets due to its long range and the fact that I can’t be blocked like your Hextech Rocket Swarm can.

Heimerdinger will generally have a hard time vs long change champions such as Lux or Caitlyn because they can easily take down you turrets while staying at a safe distance insane burst champ can also be a problem if they can kill you before you stun them by your turrets. They best way to avoid this is to poke them down with your w so you can kill them quick if they try anything. It is worth noting though that heimer can counter some burst champions that have no in lane sustain eg. LeBlanc because they can’t heal up your poke so they are consistently on low HP.

You should have an easy time vs most melee champs because you can poke them when they go into cs and this will also deny them the opportunity to try and get a kill on you.

In conclusion I think it is fair to say that Heimer has become much more viable thanks to his recent rework because his damage has become much more reliable and much much easier to hit with. He has also received a nice new ult which can easily make him a DPS monster a bursting machine or a great CC provider in any teamfight as long as he uses it correctly and at the right time. If you enjoyed this guide please upvote it and if you didn't please tell me why in the comments