Leona is a horrible solo que.

That being said, she is the best disruptor in a premade.

You are just too reliant on others to pug 4 and expect to win with her.

Leona can still win Solo queue. The key is to rush an aegis, and maybe get a locket too.
And this is coming from someone who plays Leona in NESL weekly, and does well every time.

I solo queue Leona all the time. Only once every 10 matches do I find myself having difficulty as Leona, and its always when my team probably would have lost anyway. She is exceptionally good at locking down those pesky squishies.

My brother usually plays her while I play Irelia, we regularly win 3v2's and sometimes 4v2's. One of the things I like most about her is when the enemy CC machine comes a long (say Alistar) she can shut him down while I eat the squishy's face.

Leona is good if you have a team to support, if your team disperses then she is not good.
She is a great tank support, itemize for Auras and CDR, and she is great to make it hard for people to take the tower.

I wouldn't play leona in solo q , I need to know I have at leasta 1 or 2 friends that will make use of my support.

Also you have to understand leona may be the lowest damage in the whole game. So if your team runs low damage, it may spell disaster for your team.

After over a dozen games after reading responses to this thread with Leona in Dom, only one of them went poorly, and that was sure to your average LoL random team bickering fail. The rest, even the ones we lost, went beautifully. I'm fully convinced Leona is a very strong pick for Dominion now. I was able to make a Rammus regret ninja capping several times by locking him down for so long at that point that half our team was able to show up and pick him off easily after turret damage and leona's passive.