A lot of these guys grew up in the muscle car era and haven't been able to let the Mustang go. I've seen Mustangs praised in car mags from 1992 and up, and they were really terrible cars back then. They've come a long, looooong way since then - hell a long way since 2005 even - but they still lack a lot of refinement. My points are valid, and a lot of people simply choose to ignore them.
Overseas, a lot of critics have typically trashed Mustangs and Camaros. So it has a lot to do with biases and upbringings. Me, personally, I can look past a lot of that BS and just call it as I see it. Quite frankly, the way I see it is that the Challenger SRT8 392 is the best muscle car on the market - it's the most powerful in its class, has the best interior, has a great suspension, and IMO looks the best. It has the fewest compromises in the muscle car category. If you asked me last year, I'd have said Mustang 5.0 hands down. But since then Dodge has given us a new 6-speed transmission and an all new HEMI that belts out an additional 50HP. It is simply an awesome car.
So the way I see it is: my opinions change of these cars when one is genuinely better than the other. Is my opinion fact? No. But I also don't have biases towards any car, really. I was once an extreme Porsche hater - but I am smitten by the new 991 Carrera - it looks gorgeous inside and out (finally) and the Carrera S has the power to boot. I still dislike the Cayman and Boxster, but maybe my opinions of those will change in due time, as well.