Outer ring - who uses and what are your findings

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.

I have one for my Kenwood L07D.  I don't routinely use it, but when I did use it, I placed it directly on the platter edge, UNDER the lip of the LP, so that I didn't have to remove the ring every time I played another LP, with the attendant risk to the stylus. 98% of the LPs that I keep and play routinely are already flat; for me the ring is only to add peripheral mass to the platter to increase inertia.  (I'm not saying it wouldn't work to flatten a warped LP and perhaps to dampen resonances; I'm just saying that it's an aggravation and a risk to use it that way, and I don't feel I need it for that purpose.)