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I have a whole list but i am going to hear from you.

I know some may hate/dislike me but I thought boys over flower was way overrated... And underrated for me would be Dr. Frost. It was such a good sotry and plot, could have used a bit more love line, but that's just me!
oh. your not the only one. I think so too. 
My Over rated drama is Healer. I am getting killed in the Healer page since i have started a war on Healer fans but that's what i think. 
Underrated is My Beautiful Bride. It was...i have no words but to say it was DAEBAK!
I may also get killed but i find Sassy,Go Go a lil bit overrated.It was good for its genre but it wasnt anything new.For underrated,Its White Chistmas.I hadnt seen anything like that drama before,esp from Korea

also,i definetly agree with Helaler,every friend of my recommended it to me but i couldnt pass the first ep.But maybe it just isnt my style.
Over rated == healer 
I found Kill Me, Heal Me to be completely overrated. Thinking about it, I'm still not sure how I even managed to complete it.
Underrated : That winter the wind blows, Reply 1994
Overrated: You came from the star, Healer
Underrated: Dr.Gang, Birth of a Beauty, miss ripley, may queen
overrated: Boss & Me, Boys over flowers
Underrated:  All About my Romance -- These drama was surprisingly really cute and funny, and I loved the characters.

Overrated:  Boys Over Flowers, You Who Came From the Stars
Agreed, Healer  and You Who Came From The Stars is totally overrated. I'll also second Kiss me, Heal Me and add I Hear Your Voice. 
Untouchable, is a Japanese drama that is really underrated. Nakama Yukie is really good in it, and it's a suspenseful mystery. Wish more people had it on their radar. 
Oh and I also forgot The Queen's Classroom (Korean) is totally underrated. That drama had me bawling. 
Overrated: KMHM , Healer, BOF, Oh My Ghost
Underrated: Missing Noir M, DR. frost
I've answered this question previously for a similar thread (and I continue to update/edit it). Maybe it's just because I'm a person that ADORES lists of any kind, or I'm an overachiever that goes overboard :P .....but I've actually separated it out not just what I feel is overrated, but based on the average MDL (MyDramaList) rating. If a drama goes more than 1 point (either under or over) of what I would personally rate it as, I would consider it (by MDL standards) over or underrated. But that's a separate list from dramas I feel like are actually under/overrated (which I'll also include :P):

Overrated by MDL Standards:

Kang Koo's Story - Average MDL Rating: 7.7 ; My Rating: 1
Aftermath Season 2
 - Average MDL Rating: 7.4 ; My Rating: 2
Aftermath - Average MDL Rating: 7.2 ; My Rating: 2.5
Faith - Average MDL Rating: 8.2 ; My Rating: 2.5
Orange Marmalade - Average MDL Rating: 7 ; My Rating: 3.5
Boys over Flowers - Average MDL Rating: 8 ; My Rating: 4.5
Oh My Venus - Average MDL Rating: 8.4 ; My Rating: 4.5
The Heirs - Average MDL Rating: 7.6 ; My Rating: 5

The point where I start liking the dramas, but they are still considered Overrated by MDL Standards:

Tatta Hitotsu no Koi - Average MDL Rating: 7.8 ; My Rating: 5.5
Secret Garden - Average MDL Rating: 8.6 ; My Rating: 6.5
Pinocchio - Average MDL Rating: 8.8 ; My Rating: 7
Witch's Romance - Average MDL Rating: 8.2 ; My Rating: 7
Love Me If You Dare - Average MDL Rating: 8.9 ; My Rating: 7
Full House (Thai) - Average MDL Rating: 8.8 ; My Rating: 7.5

Within 1 Point of Difference:

Noble My Love - Average MDL Rating: 8.1 ; My Rating: 7.5
Coffee Prince - Average MDL Rating: 8.4 ; My Rating: 7.5
Secret - Average MDL Rating: 8.6 ; My Rating: 8
Fated to Love You - Average MDL Rating: 8.4 ; My Rating: 8
My Girlfriend is a Gumiho - Average MDL Rating: 8.3 ; My Rating: 8.5
City Hunter - Average MDL Rating: 8.7 ; My Rating: 9
Healer - Average MDL Rating: 8.9 ; My Rating: 9
Splash Splash LOVE - Average MDL Rating: 8.9 ; My Rating: 9.5
I Hear Your Voice - Average MDL Rating: 9 ; My Rating: 9.5
Kill Me, Heal Me - Average MDL Rating: 9 ; My Rating: 10

Underrated by MDL Standards:

It's Okay, That's Love - Average MDL Rating: 8.8 ; My Rating: 10
Master's Sun - Average MDL Rating: 8.9 ; My Rating: 10
Scent of a Woman - Average MDL Rating: 8.3 ; My Rating: 10
The Greatest Love - Average MDL Rating: 8.2 ; My Rating: 10
You Who Came From the Stars - Average MDL Rating: 8.9 ; My Rating: 10
The Princess' Man - Average MDL Rating: 8.7; My Rating: 10

My actual opinion on overrated/underrated stuff?

I think the most CLASSIC, textbook case of overrated dramas are Boys over Flowers & Heirs. And for some reason sooooo many people go crazy over Faith, but...I just can't even with that show. 24 hours of my life....wasted. But a new one I want to add into the overrated category: Oh My Venus.

Dramas that I liked but didn't feel quite warranted the hype to amount the rating they got? - Secret Garden, Pinocchio, Witch's Romance, Love Me if You Dare, Full House (Thai)....All these dramas are "good"...just not as good as the rating they achieved. 

There aren't really any dramas that I think are blatantly underrated (at least none I've watched so far), but there are a couple that I'm not sure why they aren't talked about (even more than what they currently are): Scent of a Woman & The Greatest Love.
its okay, that's love is under rated? lol. That's a new one...
^^^No, I don't think it's underrated  Hence why I didn't have it in my opinion section  

It's just that I rated it differently than it being in the 1 point range. Iit missed the mark by .2 rating points so technically it would be placed in that category  But I don't actively think that it is underrated.