Sona is historically an easy to use champion. That's a large reason why she's always had a high win rate despite little cc, large mana costs and as of the rework, a contradictory kit. Now, the itemization changes make it easier than ever to use her, and I myself, someone who rooted for Sona buffs, FINALLY enjoy post-rework Sona under the condition that I build 250% mana regen on her. (Not even joking) Just the spellthief's line alone is likely to bump up her win rate by a lot simply because it made her go from very easy to use to hard to use wrong.
We don't necessarily want Sona buffed. (At least not directly) We want her reworked to actually make sense. Her kit is frustratingly contradictory. Rapid Firecannon shouldn't make so much sense on Sona to make up for the range disparity between her Q and auto attacks- she needs to make use of this every time, because unlike the power chord of pre-rework, (where you could hold onto it until an opportune time to use it) the on-hit bonus is on a three second timer. If you don't land it, it's a nerf in comparison to pre-rework Sona. If you merely aim to land Q on two enemy champions every time, it's a straight nerf. She lost her aura for this after all.
Her heal has extremely low numbers in comparison to other champions and extremely high mana costs. It's double targetting somewhat makes up for this I guess, but what I really hate about her W is that 1. It gets stronger for hp missing and 2. it comes with a shield. And extremely short-lasting shield at that. Both of these are meant to be compensation for its low numbers, and both go against what Sona uses her heal for. She wants it as sustain. (Not necessarily as a combat move) Not being able to sustain means she can't discourage people to jump on her, which is her counterplay as an extremely squishy champion. I don't see how anyone would want a shield and % missing health bonus coupled with a damage and mana nerf in exchange for the defense aura she used to have, and I don't see how anyone would think the former is more cohesive in usage than the latter. To make use of the shield, you need to take damage, usually more than all that the shield can block AND all that the heal can heal.
I bypass this by grabbing 
after finishing
, picking up
for the passive and then finishing either
or
, usually the latter since there's less AP now and how everyone and their grandmother has lifesteal. Isn't it ridiculous that that much mana doesn't go to waste on her?
As for E... well...
http://i1239.photobucket.com/albums/ff517/ryik7/sad%20day_zps1jy4avab.png
#Sad Day
Now, combine all this with having to be right next to someone to give the full effect of your abilities. This doesn't make any sense. Do I want to hang back, or do I want to be up in the action? Is grouping up really to be encouraged? What niche does she even occupy? Nami with less cc and slightly lower cooldowns?