I've been playing Swain in normals and rank recently. I feel like he is pretty strong when you are playing him correctly.

I play him in mid lane, since mid matchups are easier. they are generally immobile, so you can easily hit your E, then follow up with W and Q combo (this is how you use W). Or you zone them with W, so you can hit your E, then follow up with Q.

Lane sustain shouldn't be an issue due to your passive and manaband flow. His W and Q give you decent waveclear. Rylai is a core item, it works with your R, so you are basically slowing enemy 20% infinitely when they are under your R. I don't buy RoA. Hourglass is situational.

I play him like a mage, so I don't play him like a front line as Swain is used to be.

However I still hope riot can do something about his E and R. It's easy to miss E and R looks underwelming (at least make the reward more visible please).

Practicing Swain in rank at Gold I OCE, I lost a lot of matches before I learn how to play him: http://oce.op.gg/summoner/userName=toxic aerie

We've created a new club for OCE Swain Mains, add Toxic Aerie / 20 minutes if you're interested :D