I had a full day tour with David this past summer.  His English was exceptional.  He was easy to connect with and very patient.  This was not the typical dry, scholastic tour but a real introduction to everything that is Italian and Pisan in particular.  He was exeptional in conveying historical and cultural info of all kinds.   We had a great time and lots of laughs as well.  Since most people will only spend a day or so  in Pisa spending at least part of the day with "Pisa Explorer" is a wise choice for those who wish to maximize their travel experience.