Buffnut:
A fighter is there to shoot down enemy aircraft - that is the measure of success.
That would be the purpose of an interceptor, a fighter actually has a much broader defintion.
A fighters job can be to protect assets. ( Other planes, (bombers), factories, radar installations, ships, territory, etc.) It might shoot down enemy planes doing that job, or it might not. Better if they do, but not an absolute requirement.
A good example would be the close escort doctrine used by Yak fighters protecting bombers. Their job was to drive off German fighters, they were specifically forbidden from leaving their charges to pursue and shoot down enemy fighters.
I believe the Boomerangs, though they were not able to shoot down any Japanese bombers, did disrupt their bombing mission. That would be a successful sortie for the Boomerang IMO.
The Boomerang continued in a combat role throughout it's career, while the Defiant was relegated to a training role after a poor showing in combat.
Looking at it that way would make the Defiant a less successful fighter than the Boomerang, even though it did manage to shoot down several enemy a/c.
I don't think many would agree that length of service is irrelevant to the success of a design. People praise the Mustang, Spitfire and Me109 for their length of service all the time. I would say that longevity helps put those three planes amongst the most successful fighter designs of WWII, so the opposite would also be true.
I guess when I asked did it meet design specs, I should have said 'did it do what the designers wanted it to do?', ie home defense, bomber escort, etc. I believe one of the design requirements of the 109 was as a home defense fighter, as was the Spitfire. The Spit was successful in that role, the 109, ultimately, was not. (probably no WWII design could have been successful protecting Germany in 1944/45)
There are a lot of people who place emphasis on kill/loss ratios. A positive or negatvie kill ratio doesn't tell the whole story, but it is a pretty important bit of evidence when evaluating a fighter. Definately not meaningless.