This mod was based on taking a look at some of those really expensive $10k plus turntables and I noticed a number of them had acrylic platters with metal weights embedded into them. From what I can gather the thinking goes that it adds mass and thus helps with platter inertia and pitch stability. Also, it would help dissipate micro vibrational energy in the platter by having another material to interact with a la constrained layer damping. So I figured I could do this too on my own, since I have a good shop and drill press, etc. I laid out a 14 point configuration in a CAD program first to mark the platter with. I then purchased steel ball bearings of 5/8" diameter from the Scientific section at Amazon.com. Cool DIY site BTW. I test drilled some scrap MDF first of course, and drilled the holes just deep enough for the bearings to clear the top. I dropped in some super glue after each one to make sure they were held in tightly. I also use the Acromat on top of the platter, which IMO is great upgrade- made backgrounds way quieter and just tightened everything up very nicely.
Anyway- the bearing mod was worth it for sure. Tighter bass and highs- just everything a bit more "solid" sounding. As a bonus it looks pretty cool in the sunlight as you can see. :)