I considered the harness a starting point and it was helpful in that it included all of the real large cables that I needed FWF, and also provided a backbone for my wiring. The wires that are common to just about every airframe are there and labeled (p-leads, alt field, landing/taxi lights, etc).
I had plenty of extra wire and connectors on hand to make changes as needed. Keep in mind that I have a simple setup, used Van's wiring diagram and their switch/circuit breakers, and don't have an e-buss, dual batteries/alternators, VPX, or any of that other fancy stuff. I do have internal EFIS backup batteries that will keep me right side up should all else fail.
If you also are planning a relatively simple/stock system, the harness will save you some time, but make sure you understand it all and don't just blindly connect wires - every setup is a bit different.
Chris