...lots of people aged 12-50 are using it as their main texting and groupchat app. I have friends, family and colleagues in there and everybody I know on it uses it extensively. Also, anytime there is some event (be it sports, nights out, bachelor parties, holidays) or any type of real-life group is established (roommates, classmates, families, close friends, fraternities), WhatsApp is there to facilitate.
By the way, any comparison to traditional texting is ludicrous: with recorded voice, "I am here" GPS location with maps integration, multimedia sharing, etc. Just like most of its competitors, I'm sure.