As a person who lived in Korea for three years I will definitely vouch for the authenticity. The portions are substantial. The only major difference from Korea is how much meat they pack into the dishes. I was very impressed with how meaty the dumplings (mandu) were.
I have had Korean food in DC, St Louis and Oakland and by far Korea Garden has the boldest, truest spices. The kimchi is actually spicy and the spicy pork put a fire in my belly (in a good way).
My wife ate kimchi stew and was also very happy.
The server was very friendly and made sure my cup was always full.
Cons: keeping Korea Garden out of the five star rank is the slightly high price range, lack of parking and the servers lack of English. So speak Korean if you have any special requests. If you order "as is" they understand the numbers and you really can't go wrong.
I would give a 4.5 if possible. Delicious. If you want real Korean food in Pittsburgh, this is a must try.