Wouldnt trust Hal-frauds with a wiper blade let alone a radio!!!
My mrs works in halfords and does a lot of stuff like bulb fits and wiper fits.
She also rebuilt the cylinder head on my 500 bhp turbo engine in my nova and fitted the rs2 inlet to her clio. Likewise her parrot kit she fitted and aftermarket steroes etc too and she does lots of other work on all our cars.
So based on that you'd be pretty stupid not to trust her with your wipers or radio IMHO but as has been said in the thread already it's pointless trying to assign one value to thousands of people, they vary massively. At least if a crap one does mess something up though you know it will get put right, just like it has for the thread starter.
Local garages are just as much of a lottery really, but then you dont have reassurance things will get sorted if they f**k it up as their pockets aren't as deep as halfords.