Fair warning, I love D'addario. Everytime I try new brands of strings I regret it and end up coming back to these guys.
The 12-53 Phosphors are no exception. Here is what makes them great:
Sound: A little more mellow than your regular 80/20 which means the first hour or so of play already sounds great and you aren't battling the horrible trebly sounds you sometimes get on fresh strings. These strings punch and have so much low end!
Longevity: If the first hour already sounds great be prepared for very little change in sound over time. I don't know what magic they wound into this set but the sound consistency is ridiculous.
I really don't have any cons for these strings. When I play a lot (unlike at the moment) and am therefore not using ultra-light strings (such as at the moment....) for my poor wee fingers (feign pity here), I come back to these time and time again.