I'm not an expert on fighter jets, however even my untrained eye noticed a few problems with the Russian stealth fighter.
1) Framed cockpit. The metal frame that runs across the middle of the pilot's glass cockpit will reflect radar energy. That is not stealthy.
2) Lack of sawtooth edges. "Sawtooth edges on cockpit edges, landing gear doors and other openings also break up radar." See
http://science.howstuffworks.com/f-22-raptor3.htm
3) Air intakes are not canted. From the side, an enemy fighter that shines its radar at the Pak Fa will get a nice radar reflection. On the F-22 Raptor, the air intakes are angled downward. An approaching enemy fighter's radar waves are reflected downward towards the ground and away from the enemy fighter's radar detectors.
4) The rear of the Pak Fa looks huge (i.e. has a huge cross-section). Compare images of the Pak Fa's tail section (including engines) to the flat and small cross-section of the F-22. I don't think the Pak Fa is stealthy from the rear. If an enemy fighter points its radar at the Pak Fa from the rear, the radar signature will be large.
Bottom line: I give the Russians a grade of "B" for a good effort. However, the aforementioned four and possibly more problems will prevent the Russian Pak Fa from seriously approaching F-22 stealthiness.