Once you have figured out how the booking system works (you have to order tickets around 3 months in advance), and if you are a bit flexible about the date and time of your visit, it works quite smoothly and you can leisurely walk to the counter in the building next to the church (past the rows of tourists who have not made a booking and are told by the staff that they are full for the next weeks) to pick up your tickets (an alternative option seems to be available - book a guided tour of Milan with one of several agencies, and many of these include the church; this can apparently be done on shorter notice). At the appointed time, you walk into the church and then are left alone for about 15 minutes (audioguides are available) to admire one of the great masterpieces of the Renaissance, done by one of humanity's great geniuses - Leonardo's "The Last Supper". I found the visit very pleasant and enlightening, and well worth the effort to book a ticket. The complex of Santa Maria delle Grazie is in the centre of Milan, only a relatively short walk from the Duomo (which would be a WH site in any other country).