Just wondering, is the word "woah" a good thing, a bad thing or both?

That depends.

If you know you've gained a lot of weight since your BF has been gone because you let yourself go, got depressed and met Ben and Jerry but you think he won't notice after being deployed for the past 6 months and you go to JC Penny and buy a frilly coochie skirt and a tube top and meet him at his homecoming dinner at his favorite restaurant, "Awesome Fucking Steakhouse" or some shit and you RUN towards him when you see him, wrap your arms around his waist, shout his name, let go, and look into his beautiful baby blue eyes and the first word that comes out of his mouth is

"Whoa"

That might be a bad thing.

Eg. I shat my pants.

Woah.

Eg. I got a ferrari.

Woah.

Then there's the Keanu 'whoa' which cannot be defined.

Stop, when riding a horse.

It depends on the context. Usually I use it in a joking way.

"Woah! Calm down broseph!"

Like jucedaguy says, "Whoa" means to stop. You'd use it in other contexts when something is shocking you or anything really that you'd want to stop or slow down.

I use it when I find out something that has shocked me. Could be either good or bad.