I actually watched this drama because I saw a viewership list, and saw that close to 40% of Seoul was tuning in to watch Boys Over Flowers. Truthfully, I watched it because of it's widespread popularity. Here are the pros and cons.
Note: Contains spoilers!
Cons:
1. Goo Jyun Pyo is a jerk. Why the protagonist fell for him is beyond me.
2. You might learn some "mean" vocabulary or wrong phrases, as one of the characters says a lot of phrases incorrectly and is quite mean. However, he is always corrected on screen.
3. The characters make a lot of really idiotic decisions. However, this is what makes it a drama -- right?
4. Stereotypical drama plot: poor good girl falls for a mean rich guy.
Pro:
1. You get to see what they eat for breakfast!
2. There's a scene where they make kimchi.
3. You'll understand the sitcom jokes about the show.
4. Good looking guys and girls -- though this is almost a given with a k-drama.
5. The little brother is really funny.
6. You get to see the inside of a porridge shop.
I hope that helps!