This visualization from this Verge article nicely summarizes how OLED has evolved over the past 10 years:
I have an older generation OLED TV, but when I saw some of the latest TVs side-by-side at a home theater specialty store recently, the brightness doesn’t seem to improve the visuals a lot in a dark room. Because I primarily watch TV only in the evenings, I guess I’ll wait to see what the 5th generation has in store 
Cheers,
Kirupa
