Just how worth this Ring is?

I learned that only the left rings can be placed into units but not the right handed rings. Currently I am using the Critical Strike ring, fully enchanced into level 20 (That is the ring that I bought after getting all 3 Critical strike rings, melee, ranged and tech to 20 Level).

So, is the Friendship ring worth it?

It's a 5% damage reduction, so it is worth it if you value taking less damage over dealing more damage, with the middle ground being a Valor ring (1% damage dealt, 10% crtiical rate, 3% damage reduction).

Personally speaking, I would stick with the Critical Strike.