Watching The Heirs is like indulging in a bag of second-rate candy. You know it's not that good and that too much will leave you nauseated, but you still can't, for the life of you, stop eating. In light of this observation, my rating might seem as too favorable, so I have to mention the one quality that redeems this show, above all: I'll be damned if I say it's not entertaining.

The story is full of clichés, some of them infuriatingly abundant in other kdramas as well: rich boy/poor girl, bad boy/good girl, lots of wrist-grabbing and bullying, love against the odds, family issues... yada, yada. It falls apart at times; I couldn't quite figure out why things kept happening in the most painful way for our protagonists - other than, of course, the fact that the show must go on for 20 episodes - since there seemed to be just drama for the sake of drama. I still ate it up like a fool, though: glued to my screen, waiting to see what other plight would befall our poor duo. Sugar cravings, anyone?

The acting is adequate: as many other reviewers have observed, the supporting cast shines through the main characters' bickering. I especially enjoyed Kang Ha Neul and Kang Min Hyuk, who were incidentally two of the least problematic characters in the show. I can't say that Lee Min Ho or Park Shin Hye were bad - they just weren't anything special. (Besides, as far as I'm concerned, nobody can shine through their role if they're dressed like LMH was. Good grief, first the animal prints of City Hunter, and now those tacky dress shirts and fugly sweaters.)
The fact is, I don't believe anyone's role was well fleshed out. The "love triangle" is an archetypical one; the heroine, while showing some pleasant spazz in the beginning, is later being thrown around like a tennis ball; the male protagonist grabs the heroine from the wrist and is rather insistent she obey and follow him around (and she listens!); the third corner of our triangle is the "bad boy" of the group, being shown as a rather despicable human being but having a change of heart midway. In this role, Kim Woo Bin actually did a very good job. I loved to hate him, and although it is clearly shown he can't be forgiven for his shitty behavior, he repents and grows as a character. All the supporting actors had a story of their own, and I could sympathise with them, but it was never shown as much as I wanted; just inferred.

Since, as I said, I literally devoured the episodes one after another, I might have given it an even higher rating, were it not for a tiny little detail: the problematic aspects of the romance. A romance where the heroine almost never follows the male protagonist by her own free will, but is instead constantly forced by wrist-grabbing, as well as a romance where the heroine is constantly acted upon instead of having a say and her own agency, is one of my biggest pet peeves and it took away from my enjoyment. Some might find it romantic; I find it obnoxious and outdated. Another thing is that the love triangle never really worked for me, and the reason is quite simple: no matter how much of a development Kim Woo Bin's character had and how much you root for his happiness, one can never excuse his constant bullying and belittling his classmates and his love interest alike. I mean, even the main love interest was a bit obsessive, but KWB's character takes the cake!

The TL;DR version: Watch for the laughs, be aware of the problems. Drink a shot everytime there's wrist-grabbing, or bad fashion sense, and play a guessing game of when "Love Is The Moment" will start playing out of nowhere - you won't regret it.