USAF veteran. F-111 Cold War, then F-16 Desert Storm.
In order of accomplishments: Good => Excellent => Outstanding.
The J-20 is an excellent/outstanding effort by China, and the Chinese engineers have all the rights to be proud of what they did. Here is the deal...
In designing a low radar observable body, there are three rules to go by:
- Control of quantity of radiators
- Control of array of radiators
- Control of modes of radiation
They are not so much rules that can be broken as they are guidelines that there are degrees of obedience to them.
The sphere is the reference calibration body in radar design. Look up 'Lincoln Calibration Sphere'. The reason the sphere is used is because the sphere is the most obedient to the three rules.
The J-20, as opined by many, is not as obedient to the three rules as the American F-22 and F-35. Ultimately, the only way to know is to have all the current 'stealth' aircrafts under 3rd party supervised measurement, which will never happen. So for now, all we have is what we observe and educated guesses.