The emergence of the J-20 surprised many western analysts. Many did not expect China to develop a stealth fighter so quickly despite Russia's unveiling of the Pak Fa. My personal opinion is that the United States should be concerned about the J-20 but its not time to freak out, not yet at least. Despite its impressive appearance, there are many flaws in the current J-20 design.
Although the front of the J-20 closely resembles an F-22, it does not maintain a low frontal rcs because of its incorporation of canards. The canards give excellent maneuverability however, they are a liability in the age of stealth combat. Because canards present large angular surfaces that reflect radar waves, they increase the frontal rcs of the J-20. Even with software similar to the Eurofighter Typhoon, the frontal rcs of the J-20 will undoubtedly be larger than both the F-35 and F-22. It is unclear however, if the aircraft shown is merely a prototype and the final version may not feature canards. The J-20 variant shown may be similar to the YF-22 in terms of stage of production. This is not to say the current J-20 is not stealthy, the rcs of the J-20 is likely somewhere around .01m^2. From the rear, the current J-20 design is a disappointment. Even the most "enthusiastic" analysts agree that the J-20 maintains a poor rear rcs do to the placement of its engine and the design of the engines themselves.
For such a large aircraft, the J-20 uses underpowered engines. Despite its best efforts China cannot currently manufacture its own high performance jet engines. Thus, China must purchase AL-31 engines from Russia. These engines provide inadequate thrust for the J-20. The J-20 is much larger than most air superiority fighters such as the F-22. The J-20 is around 70 feet long (F-22 is 60ft long) and much heavier. China is likely to ask for improved AL-31 engines but this will only help so much. Even the most advanced 117S version of the AL-31 "would likely not be sufficient to extract the full performance potential of this advanced airframe." -(Kopp, Goon, 2010) China really needs to domestically produce its own jet engines and not be at the mercy of a foreign power. Particularly Russia who is reluctant to give China their best technology. China frequently reverse engineers what Russia gives them, like the Su-27 (J-11B Chinese made copy). It is projected that China will not be able to fully utilize the airframe for another decade.
From what I've read, I agree with Bill Sweetman in regards to the true purpose of the J-20. The J-20 is much larger than air superiority fighters and closely resembles the American F-111 fighter-bomber, "looks like a stealth F-111"-(Sweetman, 2011). The overall design and the manner in which the aircraft's rcs is reduced further supports Sweetman's theory. "The J-20 has a chiseled nose and trapezoidal engine inlets similar to the F-22’s and F-35’s—and indeed the T-50’s—indicating a potentially high degree of frontal stealthiness, useful for an aircraft that needs to fly straight toward an unmoving opponent on the ground. This reinforces Sweetman’s assessment that the J-20 ‘is a bomber as much as, if not more than, a fighter.’"-(David Axe, 2011). This argument is further supplemented by the fact that China has already invested in an advanced 4.5 generation fighter, the J-10. China's current fighter-bomber is long due for an overhaul.
In conclusion, the J-20 should raise eyebrows of Western analysts but the J-20 is nothing to freak out about. It is important to note that the emergence of the J-20 proves that China is progressing faster than previously expected. Western intelligence should retool they way they gauge China's technological progress. In retrospect, it was only a matter of time before China joined the rapidly growing 5th generation fighter club.
An interesting article done by Aviation Week: