anyone use a Symposium platform under a turntable?

Depending on the turntable, I would think. Symposium makes a point that they work best when there can be good transfer of vibration from equipment down to the platform thus the aluminum pugs supplied with the platform.
I have VPI Classic which has rubber feet, in this case, Symposium really degraded the sound quite a bit (At the time I un fortunately did not try the aluminium pugs instead of the rubber feet). My friend has VPI HRx that also did not work well with Symposium. On the other hand, I heard Blue Note Piccolo which has metal feet with hard plastic cap that really benenfit from Symposium.
In case of VPI with stock rubber feet, I found Gingko works better.

I use Symposium Ultra under my Super Scoutmaster with rim-drive, which is great. All around better performance, deeper tighter bass, deeper soundstage, etc. I had a Gingko under my SSM before I went to rimdrive, which was very good, but the Gingko didn't work with rimdrive, as the table and arm moved laterally with the Gingko's tennis ball suspension system. I use Symposium shelves under all of my components, and rollerblocks under my CD transport. Need to use cones or point pod plates, not rubber feet for best performance.

I use the Ultra Platform under all my turntables. It is important to notice that you need to direct couple the plinth with the platform witch is acting as a subplinth. Never use rubber or sorbotane feet. Rollerblocks work well with VPI.

http://www.symposiumusa.com/VeysVPI.html

Use SYPOSIUM ULTRA Stelth Great bass wonderful imaging sounstage etc table is TW ACUSTIC AC1 With Black Night Power supply feet and VTA Armboard. Wonderful setup!!

For my Nottingham Space Deck (no suspension) I use a Solid Steel stand with a Symposium Svelte shelf on the mdf shelf, then Symposium Roller Block Jrs, then Nottingham plinth and table. The Roller Block Jrs seem to add almost as much improvement as the Svelte shelf. Tighter, cleaner, quicker sound, keeping the slight Nottingham warmth.