Played nothing but Sol since he came out on PBE and live, here are my thoughts.

So I've played Sol enough to feel like I have a good grasp on the champion. (not on this account so don't lolking me lol) All in all I feel like he has some decent strengths but he also makes a ton of tradeoffs for those strengths. To put it in bullet points:

PROS

• AOE sustained damage
• Really high movement speed when moving around the map. Not so much in lane but he gets places faster than most.
• Q AOE stun is pretty strong if it hits.
• His ult is actually fantastic. Great damage, peel, hits AOE, slows. Lackluster in looks / theme for a cosmic star dragon but it's so effective it's hard to care.
• Can gank from weird angles other champions can't gank from thanks to his flight
• Excellent in team fights and against stealth champions because of his AOE damage
• SPIRIT BOMB MID LANE LATE GAME

CONS

• Horribly mana restrained. Without a mana item you will go OOM in a matter of seconds using W. Often even late game you'll find yourself backing after almost every fight since W taxes you so hard.
• Absolutely, positively CANNOT fight any champions with mobility in lane. He's quite strong against immobile champions but there are a lot of champions with dashes in this game and pretty much all of them give him problems.
• Cannot be first picked in lane. He doesn't even feel like he really fits into mid or top. He can do either lane but mostly it feels like a matchup based thing.
• Can jungle decently if left undisturbed but he's very weak to being counter jungled since he's not a strong duelist.
• Stars are really awkward to align and if you miss one that can mean the the loss of a fight.
• Can't detonate his Q next to himself so if someone is on you, you're literally SOL (it works both ways!) without your ult, or you have to blow summoners.
• Can't hide in bushes or just out of sight because of his passive. This is actually really annoying.

Personally, I feel like he leans a bit on the weak side. I think in the right comp he could work, especially if your team is willing to focus on team fights, but I think he's going to be a niche pick that you either use as a counter pick or to fill in jungle when the team needs ranged magic damage and it's lacking in mid or top. Despite having a simple kit, the margin of error on him is VERY low. Your spacing on his stars has to be perfect at all times and if someone suddenly flashes or dashes inside you passive, it can be pretty devastating. Positioning is more important on Sol than any other champion I feel like.

IMO if I were to change anything with him I would say reduce the mana cost on W and let him detonate Q as soon as he launches it. He's way too vulnerable as is and I feel like he's got too many tradeoffs for not that big of strengths personally. He's a lot of fun though. Personally I expect him to be played most top lane against melee champions and tanks, but we'll see how it pans out. Mid lane has a lot of stuff that beats him since long range poke and assassins counter him and that's basically all you see in mid lane, but he can still do decent against immobile mages. Maybe the mid year mage update will give him some tools he's missing atm.