I haven't seen a fake myself, but a while back someone bought one on eBay and came here complaining that it wouldn't even fly. The likeliest places the clone makers would cut costs would be in the battery, the motors and the quality of the wing mechanics; any of those will kill the performance of the clone.

Visually, the only difference was the name on the package- they used the same size box and plastic cover, same color printing, everything. Probably manufactured under the radar from the 'real' molds and dies! 

Rovio rules!
Wisconsin Iron :: Cellular Atomics