The Mustang tranny is some piece of **** built with Chinese parts or something, and it shifts like absolute dog ****. However, that new 5.0 with a T56 would solve world hunger and end all wars overnight. I'd take a new 5.0 over a piece of **** Camaro any day - even with the crappy manual box. You can fix the box, but you can't drop 400 lbs very easily.
Blah blah blah
Some people have had problems, LOTS of people ahve had zero problems. Ive been driving my '11 GT for over a year now and the transmission hasnt given me a lick of trouble.
Its only legitimate problem is that 2nd gear is really hard to get into when cold, if youre a retarded ape youll slam it into 2nd anyway and then get mad when the syncro is shot. Luckily, the ratios are so close and short (5th gear is 1:1) that skipping 2nd is very easy. Even when my trans is warm and shifting like butter I still shift from 1st to 3rd pretty often.
As for the feel, its not the best, a T56 does feel better, and a Miata gearbox is pornography in comparison, but its not that bad. The main issue is the shifter box mount which is soft rubber with a zip tie around it >_<
Once that has been replaced it feels much better.
Also, people have put some real power through the stock manual box already and they have held up well, especially compared to some of the manual transmission in the past.