I like it.

Only real problem I see is in draft - if you're playing your opponents probably aren't, so they're less likely to be chucking instants around.

Mm, this could be mono- or even mono- to give it more chance of triggering.

Is it just me? Or is this card anything but red/blue? You wouldn't think team Izzet would discourage people from casting spells...

To be honest, though, I'm not sure what colors I would make this. Perhaps just white?

I don't know what this was before, but it definitely fits in blue now.

It was originally R/B and only triggered when an opponent cast an instant or sorcery. Needed a boost.