This point isn't entirely relevant since the OP is looking at bikes with STI levers. The cheap run of the mill interrupters will pull roughly the same amount of cable as the STI. Also note that they work fine with mechanical disc brakes intended for road use.

I used to love my interrupters, but I eventually stopped using them and now have a couple of pairs sitting unused on my shelf. One downside is that if they aren't installed just right they can add friction to your system and reduce braking power. This effect is minimal if they are installed correctly, so it's more of a convenience issue than one of functionality, but it was enough for me to stop using them once I was on the fence.