Got my stiff rings installed for a few days (since Sunday)
IINM i got 3 upper (thin ones) and 2 lower (thick ones) installed per side on the front cross member
2 lower (thick ones) installed per side on the rear wheel axle
my review:
Pros:
- improved steering/road feedback and responsiveness
- despite the improved feedback, usually this leads to worse NVH (especially vibrations per say), however that is not the case, NVH is slightly improved surprisingly
- since i also got stiff rings installed on the rear, it feels more solid on the rears, most likely due to it reducing flex especially when i have worn/broken bushes (it's not making any annoying squeaks and i havent got the time to go replace them, shops all say need few hours/half day++)
- the best charm of this is, if you somehow don't like it, or if you have an accident which involved the parts where stiff rings are installed, so long they aren't damaged, they can always be removed/transferred. process is always reversible

Cons:
- none that i know of in regards to performance
- gain is very minimal compared to the price you pay, basically just less than preferable/expected price vs gain/performance ratios
*note: It could be because my car (old 1999 Proton Iswara aeroback) is on on cross members and not subframes? even the rear is just a simple torsion beam/axle, can't expect much
Conclusion:
If you have the extra cash, want the extra chassis stiffening/enhancement and better steering response/feedback, this would be a great consideration for upgrading/modding
best of all is that should you experience any downsides (again, from my experience i dont think there is), the process is always reversible by simply removing them