My personal favorite champion for support role. She has strong harass skill and decent heal, if played correctly you can zone out enemy champion from farming. Main challenge for playing her is you need a lot of practice to know how to use her passive and Q for harassing safely.
I noticed that with Magic Penetration Runes and Mastery, you can do around 100-150 damage from her passive and Q to enemy champion at level 5. Combine it with her ultimate and your AD carry damage, you can get a kill without jungler help.
I think she is a little bit overpowered with correct play style. For more details click here for my guide on solomid.net