Outer ring - who uses and what are your findings

Best way to flatten records is an SME style of system. Screw on clamp with a specially shipped bottom and a washer under the record.

25 or so years ago I had an Ariston TT, the "minimalist" round one (RD40?) which used that method. Very effective. Since then, I've made very simple mods to a clamp that accomplished the same thing: total flatness. It must be a clamp, not a weight, as it needs a positive grip on the spindle to hold it down.

Count me as another user of the VPI peripheral ring along with a Stillpoints LPI. Very effective or I wouldn't use it.  Like @lewm I mainly got it to add mass to the platter, but I can't help but think that if you like your platter mat then bonding it more tightly to the LP with a ring weight is a good idea. Is it a PITA to use?  Yes.  But in all honesty I'm not sure if that's a feature or a bug.... 

one thing that does truly irk me about it: fiddling with the weight to get it to sit evenly on top of the LP.  

I use an outer ring  from TTWeights with my VPI Scout when the LP is not sitting flat against the platter. I wouldn't give mine up but at today's rate of $1K+ for one it's cheaper to get a new record, if possible, or have it flattened. On a record that's not laying flat with just the clamp the outter ring makes an audible difference to me. 

I use it on my Clearaudio Innovation; the matching ring fits perfectly so no fiddling with a centering spider/etc. I’ve made it part of my routine and never go without; it’s worth it, never having to worry about edge warps again. This particular table's platter is so massive, that the added flywheel effect is negligible.

I think it enhances the sound subtly (nothing major on otherwise flat records), but my main reason for its use is to combat warps. I also have a SOTA Nova V for a 2nd table, and its vacuum hold down is also very effective!

Systems that rely upon a center clamp or weight, such as the SME one, certainly are superior to doing nothing, but if the LP is truly warped in a nasty way, you cannot flatten the outer edge of the LP reliably just by clamping it at its center, no matter how artfully the platter is dished, etc.  And if that system works for one side of such a badly warped LP, then it is unlikely to work as well when you flip to the second side.  So, for really badly warped LPs, if you cannot bear to recycle them, I think you'd need a peripheral ring weight. In my experience, warping always affects the outermost grooves the most.