The Santos is a great introduction to syphon brewing, with all its joys and frustrations. I have been using mine on the stovetop for a couple years and it seems I'm still learning how to best use it. There are so many variables, including the volume of water, the type of filter, the temperature of the water when you set the top in place, the time you let the coffee stay "north", the fineness of the coffee, the regularity of the grind.... Need I say more? This isn't unique the to Santos, though.
I've had the best success brewing at 3/4 capacity. Any more and the trip south seems to get irretrievably stuck sometimes. A cool, washcloth usually kickstarts a stuck return trip, but not always. I'm certain that the single most important element of successful brewing in the Santos is the grind of your coffee. Just like any other brewing method, the grind will affect the flavor, aroma, and brew time of the coffee; however, with the vacuum method, you must also concern yourself with clogging the filter. I use a manual burr grinder and have found that adding a few drops of water eliminates the static and promotes a more even grind, which has greatly improved my success rate.
The Santos uses a plastic disk for a filter. It's okay, but not perfect. You can use almost any of the other syphon filter systems available on the Santos, including a Cory rod or Hario cloth filter. I don't like the Cory rod in the Santos, as it makes the whole unit shake violently if let over too high a flame. It's also more awkward to clean since the rod falls out, unlike the other filter mechanisms which are held in place by a spring. The Hario cloth filter works well, but is more bother than the plastic disk.
The Santos comes with a few cheap plastic accessories. The stand is worthless. Throw it out. The holder for the upper pot is cheap and ugly, but helpful to have something to hold the hot bowlful of grounds at the end of the brewing cycle. The remaining liquid can drain into the holder as it cools. I had a small problem with mine, however. I left it on the stove and it partially melted one day when left too close the to the oven vents at the rear of the stovetop. Still works, though.