I main Janna and I was excited for a new support, but so far my tests find her more suited to a... I can't believe I'm typing this... "Attack Speed AP Carry Support". A weird combo of Shurelia's, Boots, Aegis, Rageblade, Malady, and WotA makes her do some pretty crazy dps while also being able to buff, cc, and otherwise do the support thing. Heck, Whimsy gives AP so she can make herself to MORE damage, and when you get the hang of that skill shot she's freakin' insane.
That said, she's a LOUSY support when it comes to going with the current meta. Its far too easy to snag CS and kills with, and the fact that most supports run with an AD carry bot makes her buff a little less useful than Janna's +AD shield.
But hey, if you're looking to try something different and have fun, break the mold, etc, you could do far worse.