In the current version, in my opinion, Phantom Lancer is the most "broken" hero, perhaps second only to Magnus. His biggest weakness is a lack of early anti-push, but this is true of many carries. He has a lance to spam giving him laning presence; with prudent item choices and the buff to agility gain he can have early and mid games, in contrast to a two big-item late game carry like antimage; as a hard carry he loses to none; etc.
Funnily Azwraith is strong against heroes many consider counters to him: Bounty Hunter, Slardar, Naix. They may have the early game on him, but survive that and PL is a monster against single target carries such as the aforementioned and heroes like TA. He also excels at disrupting the back support-line, even without the now-defunct radiance.
He is vulnerable to heavy AoE magic or physical, but the way he is designed he has ways to deal with both. He has built in magic resistance and as you fight with your illusions you add tankiness anyway. Against a physical AoE such as Gyrocopter, likewise you are fighting with your illusions and if you have sufficient anti-push you let out an incessant and relentless stream of illusions. With diffusal and the buff to agility-gain, he has more than adequate dps and all you have to do is tank up and have a way to keep your illusions spawning.
Skill build:
My favored skill build is QstatsQWQRQ and then points into E. Sometimes if I'm in a contested lane and Doppelwalk is unlikely to be useful I will put points into stats instead. It is a mistake to put too many points into stats and Doppelwalk because you won't have Juxtapose maxed early enough. Juxtapose is like glaives on Luna, or cleave on Sven--it is your main pushing power and fighting ability and you want to have it maxed in time.
Afterwards I put points into stats over Doppelwalk.
Item build:
I favor Tranquils > Ring of Aquila > Wand + Drums > Diffusal > Heart > Butterfly and then decide from there.
Tranquils, Magic Wand, Ring of Aquila, and Drums is a stable early game build order and with this you aim to treat yourself as an early-mid game capable hero. The enemy team must invest great resources to deal with your split push, you can team push, etc.
However, if you are paired with something like a KotL and you view your role as the mid-game carry, it may be wise to Tranquils > Drums > Diffusal. You should still get drums for stability. It gives you base stats you need once you have diffusal anyway, and the item is based on endurance aura so your illusions benefit.
A few notes on other items:
With treads you sacrifice the cheap sustaining power of Tranquils for the option to treads switch and +8 stats. Your illusions will not benefit from the attack speed and I do not recommend this item ever on PL. Late game you will switch to Boots of Travel.
Manta Style is at face value a very good PL item, but I prefer to get it after Butterfly, if at all. Perhaps for some reason you really need to create illusions quickly and fight early on, then you can consider it after Diffusal but this will just delay every other item because you still need to Heart into Butterfly.
Daedalus is a traditional item. Illusions will get critical strike but will not get the + bonus damage, making this item only a last slot consideration if you need to fight enemy carries head to head. In those situations where you absolutely must be able to 1v1 the carry, you can also consider an Abyssal Blade for the active. However with Abyssal there is no benefit to illusions.
Eye of Skadi was brought up during iG vs. LGD.int discussions when Zhou went double heart. Indeed from a stats perspective, it is not sacrificing unacceptably much in order to get the benefits of Skadi, and in the super late game if given the lineups all you have to do is beef up, this may be the better choice over a second Heart. To use Skadi on PL with Diffusal, you just need to pick it up first before Diffusal. However in order for illusions to still mana drain, Diffusal needs to be in an item slot before Skadi.
Soul Ring is a good alternative in a contested lane as well so you can spam people out.
Now because of the nature of this hero, he is often run as a single core and opponents often reply with multi-cores. For example you may be the only carry on your team but the opponent may have gone with Gyrocopter and Tinker, or Luna and Furion, or some combination of that sort. Therefore it is a mistake to drag and farm till kingdom come, thinking that you are Azwraith the Phantom Lancer and no one nor his mother is out-carrying you late game. The usual principles of DotA still apply. You should push when you can and when you have an advantage you should maintain it and capitalize.
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