After a glorious 3 week trip through Peru and Bolivia, we had 10 hours to spare in Santaigo before our midnight flight back home. I can think of better ways to spend it than on a bus ride around Santiago. The city isn't the most photogenic, it was cold and occassionally wet, and there were student protest marches so we didn't get to see the whole route. My suggestion is to find something else to do on foot if you find you have a few spare hours in Santiago.
A great day and well worth the early start. The train to Aguas Calientas was very slow, with great views of the countryside, valley and mountains, but the ultimate was the first view of Machu Picchu. Coming through the gate onto the terraces was very a special moment. Our guide gave our small group a very informative running commentary, and our 3+/- hours at the top was well worth the long day. Lunch was great back in AG and the just as slow train ride back to Cusco was pumped up by a Peru Rail fashion parade. Yes a long day but would do it all over again in a heart beat!