They are pretty smelly.
I use them in obvious places to knock down vibrations:
* between the BB platforms my two turntables sit on and the rack's butcher block shelving
* between my subs and BB slabs they sit on (subs came without feet or spikes)
* under the monitors that sit on a wooden dresser in my BR system
My 150 lb floorstanders (designed to be used without spikes, and none included) sit on carpetting. But if they sat on wood floors, I'd try the blocks under them as well -- just to keep LF energy / vibrations from travelling thru the floors.
Also use them anywhere I want to create space between stacked gear, and to prop up heavy power cords at the IEC outlets. Limitless uses. So many in a box...and so cheap.....I just use them. Even for non sonic purposes like propping and spacing, etc.
Do they improve the sound? Damned if I know. I don't get anal about this tweeky stuff, and I sure as hell don't A/B with and without. So long as I don't hear a detriment upon insertion, I go with it. My attitude is use it "just in case" it does improve things.
I can say this....
I can jump up and down in front of both tables, and I can knuckle wrap pretty hard on my DIY rack and neither table skips a beat. Also passes with flying colors the standard Fremer tap test with needle in groove of non-rotating platter. And my subs don't rattle and rumble thru the floors, keeping my neighbors happy.