First impressions on Swain
First off......and I've said this MANY times so far, but......I fricken love what Riot has done with Swain. The detail, the lore, his theme.....just.....everything. But after having gotten my hands on him 7 times just today, I would like to tell you what I think of his kit.
#Passive Swain's new passive is kind of similar to Yasuo's Q in that in order to make the best use out of it, you need to master the rest of his kit. Yes, you can pull people towards you that's been CC'd, but the only reliable CC you have is your E, and we'll get to that later.
#Q I think his Q was a little overrated by the community. Yes, it can deal a crap ton of damage, but you do have to manage it wisely. Despite the mana tools we have available to use, if you're constantly spamming his Q, not even the mana return on it will keep you from running out. It is a very handy tool to have, but it isn't as OP as you may think.
#W If Swain ever needs a buff in the future, it's possible that Riot could look at this first. The reason being is that it is not reliable.....at all. The vision radius is very small (think of your vision radius during a Nocturne ult), which isn't too big of a problem, but I'd say there is certainly room to make it bigger. Despite the damage numbers on paper, it really doesn't do that much unless you build full glass cannon......which you shouldn't. And even then, you should max this last, especially since the mana cost ramps up on it pretty damn fast.
#E This is where good Swain players are separated from the great Swain players. Not too often do you have an ability where the return on the skillshot is more important than throwing it out. It can be deceptive both to your opponent, and you. However, a nice thing about it is that if you his E at max range it's essentially a straight up snare. So, being able to figure out the distance on his E is crucial to mastering him. If you throw it out when your opponent is right in font of you, you give them enough to time to step to the side and dodge it. Yes, you can follow them, and try to snare them that way, but that is easier said than done. So, why not just remember the range on it, and give them as little time as possible to react to the snare.
Also, since it is the only ability that works with his passive, pulling them in after a successful E will actually put them just in range of your Q, allowing for some pretty impressive damage.
#R Swain can no longer 1v5......in fact, it's difficult for him to even 1v3. Whenever you play Swain, remember that you cannot even activate you ultimate unless you have at least 1 soul shard, and you can only get them from either your W or E. Since your W is not very reliable at all, you have to rely on your E in order to get those shards, so, mastering his E is even more important.
That said, I've learned that Ghost is fan-fricken-tastic on him due to his new ult. It allows him to easily stay in range, and keep draining until he can get the second cast off. Normally, I would recommend not hitting the second cast of your ultimate unless you have a low health enemy near you. Otherwise, keep it up, because the sustain you get is quite noticeable.
#Champs he counters From what I've seen, pretty much any close range mage, or melee fighter/assassin he tends to perform well against. This is mostly due to his E, since your opponent needs to get up close in order to farm, making it very easy to land your E's.
#Champs that counter him So far, I've had the most trouble against traditional mages. Their range allows them to kite you all over the lane, making it difficult to land any kind of reliable poke, or that crucial E.
#Items
I've come to find out that starting 
and 
is actually a very decent start on him. Your Q gives you enough mana sustain that you don't really need 
. And due to Dark Seal's passive, you get far more out of Refillable Potion for far less gold. Not to mention, if you can get a kill, it only gets better.
As for a main build, I've really been building him the same as he used to build. 
and 
are all still fantastic buys on him. For boots, however, I've been leaning on 
since he can be so easily kited now.......unless your a god with his E's.
That said, CDR does feel like a far important stat for him to have now than what it used to be. That said, we do have an AP item pass coming next patch that does give more CDR, so, i think he'll be fine.
#Runes His champ spotlight suggested Phase Rush, but Comet still feels far better, especially if you're using ghost. Mana Flow Band is a must since it pairs so nicely with your Q, and after that I've been going Celerity and Gathering Storm (Celerity pairs very nicely with Ghost.....again).
Secondary tree, I've been running Perfect Timing (League of Stopwatches, anyone?) and Biscuit Delivery. The Biscuits are especially helpful when you have The Dark Seal.......they can give you a pretty hefty amount of health back. Not to mention, potions and Biscuits stack, so, if you down a potion, and eat a biscuit at the same time, you'll have some pretty insane health regen for the next few seconds.
#Conclusion Like I said earlier, I love EVERYHTING about this new Swain. If I am to give one critique, though, it's that his abilities don't feel very impactful. What I mean is that lack any kind of significant punch. Take jhin for example.....everything about his kit feels like it hurts. When he fires that 4th shot, you, your opponent, and both champs feel it. Swain lacks this....most, if not all of his abilities feel very.....subtle. I'm not sure if this was deliberate or not, but....yeah. They could use a little more oomph in how they feel.
Other than that, he is FANTASTIC. He is n't a champ you can just pick up and play right away. His skill floor is kind of up there....probably on the same ranks as someone like Syndra or Kha'zix. You do need some sort of idea of what you're doing in order to perform well....or else you will easily get your ass handed to you.
On the other hand though (no pun intended), his skill ceiling isn't that high to where you have to put in some serious time in order to master him, like Azir or Yasuo. There is still plenty of room for improvement, but for someone like me who isn't very good mechanically, his skill ceiling doesn't seem too far out of reach.
Anyway, thank you Riot for this absolutely amazing rework. I want to main this guy, so, I will go back to spamming him in normals for the time being.