I've used it, and it turned out nice. The lemon zest is a nice, gentle exfoliant, too. However, it did turn the batter yellow. I didn't expect it to change the color—I only expected yellow flecks. I was surprised. You might have the same experience. I imagine if the lemon zest is dried out, it might not color the entire soap and instead would leave yellow flecks, but I haven't tried it.
Actually, if it did, I'm not sure because I used a lemon verbena fragrance anyway. I wouldn't imagine that if it did scent the soap that the scent would stick based on lemon zest alone, but it's worth trying.