I like the new Sion. However, he is a hidden power and is extremely difficult to utilize well enough to be worth picking.
By that, I mean his strengths lie in places that extend beyond his own skillset. Namely teamfights, especially in 5-man teams.
Unless your team makes proper use of Sion's new cherry-shattering CC onslaught, he won't be effective. His damage can be unsuspectingly high at times, but overall his burst is not all that great. His sustain damage is poor. His kit is not suited for dueling, cleaning up, and his split push is all too mediocre and risky.
He can't even engage well against good opponents.
I suggest placing him in teams that rely on teamfighting to win, as I already said. Strong initiators like Amumu and Ashe work well with Sion. If your team's chemistry is particularly good, combining Sion with Nocturne creates a killer 1-2 ult combo.
Spoken another way, you can think of Sion as a CC amplifier. Like Yasuo, except much more difficult to use. Charge up your skills against champions who are already locked by CC to increase the landing chance and impact. Also, stay with teammates at all times, so that you don't get caught out (Sion is slow and can't duel!). To take that suggestion to the next level, I recommend going support so that you can be with your ADC even in the early game.
Here's a tutorial for Support Sion: https://www.youtube.com/watch?v=isoV2Xdrqpk
And here's a good example of a Nocturne/Sion initiation combo: http://youtu.be/tveyldeuI_8?t=1m49s
Sorry, I don't think he's all that great as a top laner. His E harass is a killer against 0-sustain tops like AD Fizz, but otherwise he's too easy to poke down. If you're fine with hugging tower and scraping by with just CS, then that's all good and dandy, I suppose.
That said, I truly think Sion is currently best as a support (as wonky and unorthodox (and difficult) as his supporting is). I got to Plat V in solo queue with Support Sion during Season 4, if that means anything.