Well.. Initially it was a throw pick whenever my team instalocks or does something dumb like double mid, but I ended up winning every game I went into. This is mainly because

a) She can heal your support while getting it back through lifesteal
b) She can heal off opponents as well with Q and Lifesteal
c) Her range is actually very decent for both her autoattacks and abilities.

I actually saved my support and myself many times by spamming everything during 4-man ganks;
by using my Ult, W and heal.

Like this your support can go a tank as well so your team has 1 less squishy but still have heal and damage. *Cough* Better than Sivir *Cough*

Soraka only drops off very late against tanks. She can still beat aquishy midlanes/supports/adcs late due to them underestimating your burst.

Remember to also use your Q and E to trade late when mana is less of a problem.

Basically soraka works with any support, but the most broken combinations are supports that also have heal or heavy poke.

Lee sin, Sona, Karma, etc. When they go to poke and trade, heal the sp to win the trade. For sona or other champs that have heal you can heal each other in lane the whole way.

You may or may not want to try this in ranked. I know in NA servers they're a lot more strict with banning people, so please do this with discretion.