An ADF (VOR) will be a handy thing when the GPS signals stop... for whatever reason.
How many times has this happened in the last 10 years?
In 500+ hrs in the Bo in the last 4 years, I have NEVER seen the Garmin 430 loose its satellite signals to the point where it ceased to provide accruate position data.
Garmin III Pilot - yes; Garmin 296 - yes; Garmin 150 - yes; Garmin 496 - not as yet! I suspect they were mostly antenna issues.
I would put another TSO's GPS in my aeroplane before I would waste money replacing the ADF.
SDK 