Feature:
D'addario likes to point out that their strings are 'Better by Design', and they are. The hexagonal center literally bites into the wrap, melding the two metals like no other string manufacturer can.

Quality:
Always just as bright and corrosion-free as the day they were made. The Dean Markleys just didn't compare.

Value:
In short, buying any other brand in bulk is nothing more than a waste to me. Buying single sets at a music store is far more expensive, and there's still no guarantee that your strings are fresh. D'addarios, unlike other brands, will still be worth their price 2 years after purchase when you break a string 2 o'clock in the morning at your local venue.

Desirability:
Nothing else meets the same engineering standards, nor does anything else sound the same (hey, all brands are different for beter or worse).

Sound:
D'addario strings have been the only brand I've used for at least the last five years. I'll admit I used to enjoy Dean Markley Blue Steel, but you can't really expect to buy a case or seven and have them all be fresh a year later, D'addarios are always just as pristine as they day they were encased in their oxygen-free polymer mausoleums.

Support:
I've never had problems with any of my D'addario or Planet Waves products, hence service was never required. This is a testament in itself, I'm still using the same Planet Waves guitar cables I bought about 5 years ago and they're still immaculate.

Overall:
Still going strong with D'addario and no plans to ever change brands.