I am looking for the best (and easiest) way to stack a cake. I have found many posts that rave about the SPS system, but I have a couple questions/concerns - 1) can you see the plates between the tiers (I would want the tiers to be flush) and 2). my tiers are sometimes higher than 4 inches, and I've heard they are hard to cut. Any tricks to get over those issues? Or maybe a better/easier way to stack?

You're likely to get as many different ideas on construction as there are decorators.  Some swear by this method, others that one....

That said, I have not used the SPS System...though lots of ppl here do....but to answer your question "will it be flush" I'd venture to say not perfectly...the plate has to sit on top of the icing of the cake below unless you hollowed it out first....though the plates are quite thin and your bottom border should easily hide the gap.  As far as cutting the pillars, a good serated knife or small saw should easily do it.

As to my method, I use the dowel and center pole technique.  Unlike some people who drive the dowel down through the stacked tiers, I already have my center pole glued into the center of my base board and then simply slide the tiers down over it.  The advantages to this method is that you are not trying to hammer a dowel through your boards and because the hole is already cut in the center of your boards, the cake is perfectly aligned as you stack the tiers.

I use a 1/2 inch thick foamcore base board which I order from www.cakethings.com with the center hole already cut.  I use the 3/8 thick foamcore boards to ice my cakes on (I cut my own, but you can order them pre-made too from the same place).

Once the cakes are iced and doweled, the cake is ready to stack.  I stack mine before delivery but you can easily stack on site too...this system is very sturdy and if you deliver your cakes chilled you can travel long distances without any worry of the cakes shifting.