Al,
I use the 6 stage ro/di and make thousands of gallons before the TDS starts to get above 10... My good old Lake MI water comes in at 180+ TDS.
the nice thing with the DI crystals/beads is you can see them start to change colors and you know about how much life you have. I change my prefilters about every 3 months or so and I let my unit run 24/7.
I use the aqua safe system on ebay until RandalB gets going again and I can have him help me with a "big boy" system =)