There definitely is a bias, which can be expected as half the show is their jokes and such, so obviously the commentary needs to be potent.
But keep in mind, they do also tend to humourously make fun of other European countries, and they tend to favour english cars. Like when they compare an aston to any other real performance supercar, which would lead the aston in its dust. They often also dismiss perfectly capable cars simply because they think it lacks 'soul', which is very subjective. So you can't take everything they say seriously.
And finally, they have given positive review of American car (i'm thinking the z06), but realistically Detroit does in fact need to turn it up to compete, this is reflected in car sales worldwide, with Toyota now taking pole position.
@ borhor: only one more episode??? i didn't know. Damn i hate how they're totally random length seasons, particularly if they're short. Hopefully fifth gear will be on air soon.