We've eaten at Zefferino's twice. Both time at lunch, both times because they offer a three course lunch for around $20.
The first time wasn't bad which is why we went back on another trip. The last time we were pretty disappointed and the service wasn't too great.
We prefer Canaletto also at the Venetian. Better service, better food,more extensive menue a bit more expensive but worth it.
I'll agree that Canaletto is very good. We had a very enjoyable meal there last time we were in town. Also, very good service and a respectable wine list. Had the best risotto of my life there in fact. Have not eaten at Zefferino, but have heard mixed reviews, as stated above.