I am basically a romance fan but I hate watching dramas that are so old they look super fake. I watched a few clips on youtube and it didn't seem that bad. However I have been reading mixed reviews like it drags on for to long, the plot sucks, the ending was bad... All I want from a drama is great romance and a happy ending without much angst in between. For those who have watched Goong, does it deliver that? Is there anything that I should know that could be disappointing before hand? Thanks! 

Decided to watch for the third time (Princess Hours Thai remake was dragging and wanted to compare).   I am a sentimental romantic so I will tell you that the Korean version has its boring moments but still excels in entertaining you  This is because of the intrigue of deception aspect of the plot you won't give it up.   Since the main characters are "underage" you get puppy love type romance before you reach the deeper aspect of what a marriage should be and what respect and earned love is in a marriage.   The ending wasn't bad - it was just a normal conclusion (unless the viewer wanted 6 more episodes with little substance)   I would recommend viewing the drama and forming your own opinion since each writer's opinion is based on their own life's experiences and viewing enjoyment.  I would rate this one a 8.5 and remember this is an older drama.  
Honestly~ it isn't that spectacular. But it's still a nice watch. And it has a happy ending.

all I'm advising you to do is watch it without too many expectations tbh~ 
I agree with it's a nice watch, the palace intrigues were interesting and the love does moves gradually but it delivers at the end. I also advice you to be careful since the second lead is one that can give you Second Lead Syndrome.