How effective is the sponge when used with the pill?

My girlfriend is already on the pill and I always use a condom. We are considering using the sponge and losing the condom. We have both been fully tested and cleared for STDs, so we're not too concerned with transmitting them. Anyway, we knew anyway that using a condom and a pill almost guaranteed pregnancy prevention, but we want to know how effective it will be to switch to a sponge.