I personally like Clairvoyance...
I used it in matches from Bronze all the way to Masters in Twisted Treeline (Yes, it can be useful in Summoners Rift as well ) long ago on my first account
People say it is useless, but in the right hands and usage, it can be pretty damn good.
A global range with a 1 minute cool down (54 with Summoners Insight maxed) so you can use it more than Scrying Orb, but I do know that the upgraded Scrying Orb outclasses it, and if you DO get Clairvoyance ; Chances are you will lose ''on demand'' kill potential depending on the lane and role you choose.
It's a damn shame trolls make the spell look even worse =\