Suspensions on turntable...really effective?

I'm not quite sure that I fully understand your question, but I will say that a good suspension will definitely isolate a turntable from footfalls and other vibrations.  A stunt I use to prove it to non-believers is to hit the suspension of my Basis Ovation turntable with a hammer (covered with cloth to keep from marring the suspension, of course) while a record is playing.  None of the energy from the hammer is transmitted through the speakers.  While my prior (non-suspended) turntable, a Well-Tempered, was pretty good at isolation due to its mass, it still could be affected by vibrations from heavy footfalls--it clearly could have benefited from a device like a Vibraplane or Seismic Sink, but they cost more than the table and arm itself.

Compare a Sota Jewel to a Sapphire.  They can sound almost identical.  However, the suspension in the Sapphire can definitely help if there is any movement at all.
However, a bad suspension is worse than no suspension at all.  I had a Thorens 125 with and SME arm and it was so bouncy that the hanging weight on the arm kept getting tangled...

I've used SOTA turntables (Sapphire Deluxe, then Cosmos IV) and have seen how they really isolated the playing from footfalls, etc. in a "medium" bouncy wooden floor setting.  I now have my Cosmos sitting on a Herzan active vibration cancelling base (yes, the type used to isolate scanning electron microscopes), and it just transforms the table into a much more neutral device and improves the detail retrieval.  The Minus K product mentioned above is also excellent for this.

+1 for Sounds of Silence. A Vibraplane will make a positive difference in any system. Alternatives are Minus K, or at a much higher price Herzan.

Steve at Sounds of Silence is great to deal with.