I used to hang out with people who ONLY had gravity bongs. Always got me really really super fucking stoned, including the highest I have ever been in my life.
I've made a few gravity bongs in my life, but only two were just freaking amazing. I don't prefer gravity bongs over bongs because well hell my bong is a lot more convenient than the huge bucket, diffused downstem, and a bowl all put in one the large smart water bottles. That thing hit so great. I made another one with a smaller water bottle, but always had this big bucket. Which took forever to filter my tap water to make it clean. If I was able to get that bucket full of water, and have a clean large smart water bottle at all times then /maybe/ I'd prefer gravity bongs.
I prefer the gravity bong because of its efficiency. I find that I get far more baked on a smaller amount when I hit the gBong. To me its the same as shotgunning a beer instead of drinking it slow. Super effective so to speak. My gBongs are all glass too, I don't condone smoking from plastic. If you would like to see my collection you can visit thegbong.com