I've got a ring and necklace that say they improve Smithing 12% each. I've not seen any evidence they work at all. Or rather, how they work. Are they supposed to increase my Smithing level 12% each? Or by 12 levels?

I tested it by forging items with and without the gear, and there was no difference in weight, damage or value. I then improved the items both with and without the gear equipped, and again, no difference in their stats. I only have the Steel perk right now.

After reading on the UESP pages, it's possible I happened to test the enchantments at a level where they didn't offer any benefit, so I need to retest.

Are these working for everyone else? And does "+12%" really just mean +12 levels to Smithing skill?

It merely increases how much more you can temper your weapons. At low levels with those enchantments, you won't see any difference.

You would be better of destroying the ring and learning the enchantment. Then get a grand soul gem and put the enchantment on a new ring to increase the power of it. Then you will really notice the difference when you improve a weapon or armour.