It really depends on what you are doing. I originally bought the RO125 and ETS 125. The RO is awesome, but I didn't think that the ETS was any better than the Bosch and Porter-Cable that it was replacing. I had an opportunity to pick up a RO150 and ETS150/3, and it opened up a whole new world.
I had a Rigid 6" sander before my Festool plunge, but was never impressed. I used the Bosch and PC sanders, and left the Rigid on the shelf.
If a 5' sander is all that you need, I would pick up the RO125 and probably the new low profile PC.
If you are working with larger panels, the RO150 and ETS150/3 are an awesome combination.
Before finding the "system", I had pretty simple rules when buying tools.
#1 always watch what Bosch is doing. This is the one company that could change my pre-conceived notions about who makes the best tools.
#2 when buying a router or sander, I looked at Porter Cable. PC also makes some spectacular specialized tools. In addition to numerous PC routers, I still use my 4" trim saw, dovetail jig (which will soon be replaced by an Akeda), and biscuit jointer on almost every project.
#3 when buying a saw, I looked at Makita. Many will argue here, but I have a 5007NBK that has outlasted 7 other circular saws (PC, Dewalt, Milwaukee, etc). While I don't own the Kapex yet, the LS1013 is still my favorite miter saw. I have the new Milwaukee with digital readout, but still love the Makita.
#4 when buying a drill, I looked at Milwaukee or Hilti.
The system has turned all but #4 on their ears.