I started with the Morimoto HID Kit for the Sierra when I first bought my truck. And while there was a definite improvement, after having a HID Retrofit on my 01 Denali, I ended up purchasing an extra set of headlights (when they were going for roughly $600 a light), and did the Projector Retrofit with the kit from Fast Headlights... Now the headlight assemblies are much cheaper, so doing a Retrofit is much more affordable. The stock projectors are designed for halogen bulbs and simply do not compare to a HID projector. It's like putting LED bulbs in and there is no clean cutoff which causes glare for on-coming traffic.
Over the last 5 years, I've replaced the ballasts (two times), HID bulbs (3-4 times), and harness (once), and I'm getting ready to replace the bulbs again because I have one that doesn't always fire when the lights come on automatically.
I researched to see if there was a LED substitute and found that there are Morimoto LED headlight assemblies available at $1300 a set. After reading some reviews (HID vs LED), I'm sticking with my HIDs since the review indicated the beam pattern travels longer than LED.