Corsair was the first RAM I ever used to build a pc. Quality stuff allowed me to overclock my Pentium 3 450 to 558mhz! Corsair will always have a special place in my heart.
I'm running 6GB of triple channel Corsair RAM in my system right now and it's super-fast. Corsair kicks a** for the same reason Chuck Norris does: simply because they know how.
Compatibility, overclockability, heatsink desgins, customer support... overall peace of mind that when you slot their RAM into your machine it will boot up, or they will do what it takes to correct any issue.