Let me start, I have two DIY diffusers - one on my Old E and one for my Small Opal.
On Old E - all it did was silence the bubbling sound. Didn't notice any increase in smoke or taste. In fact it became annoying and we stopped using it. I like the bubbling sound.
On Opal, it does make a difference. Old E is 33", Opal is 17". It cut back on the bubbles in both cases (which helped prevent water in the smaller hookah's hose) but in the smaller hookah I have noticed a smoke increase and a flavor increase. I was mildly surprised.
I will post my instructions for making one in the DIY section - as not to distract