So much melodrama, I thought it was a comedy. Seriously, this was such a fun show to watch, and usually at the dramatic parts, I'd smile and laugh. :D

Story (7.0):
Over the top, ridiculous, and I loved it.I'm a sucker for secret identities, twins pretending to be each other, bad guys almost killing good guys, hehe. Plenty of that in here. So much melodrama.The bad guys are way too bad, the good guys, probably way too good (nah), the ending was not at all realistic, and lots of people turned about face without much explanation.I confess I watched a fair amount of this at x2 speed, and used the 10 second skip liberally, but I still loved the plot. It was so funny hilarious and drama hilarious. As in a lot of the drama was so contrived and unrealistic that I enjoyed it without too much worry.
Plot hole unrealistic galore, so much fun.

Acting/Cast (7.0):
Over the top, ridiculous, and I loved it.
Seriously, I thought most of the parts were over acted, especially Yoon Se Ah (thought that's not new imho), and it was really enjoyable x)
I also felt the twins were overacted, but Lee Da Hae did a VERY good job acting the two twins. You really forget that it's one person doing it.
I think the most organic acting in here was from the two male leads Ryu Soo Young and An Woo Yeon, and from the kid.
Still, I think all the acting, while not spectacular, fit the drama and was definitely overacted (as opposed to underacted).

Music (8.0):
I probably overrate most drama soundtracks, because there are few that I actually love and I still love the dramas; this ost again isn't anything spectacular, but there were a couple songs I might actually consider listening to outside of the drama, so that's definitely a plus.

Rewatch Value (10.0):
Okay, even if I really love a drama, I usually can't imagine sitting down and rewatching it again. Especially if it is really emotional. I really enjoyed Mother, Goblin, etc, but I have yet to rewatch them completely. Favorites, yes I can go back and watch my favorite parts.
But this drama was just so much fun, I think I could go back and rewatch the whole thing xD. (EDIT: I literally did this, rewatched the whole thing in a matter of a couple days, so I've increased this to a 10 lol. I think I had even more fun this second time watching)
I'd still probably have to skip a lot of the bad guy stuff now that I know what's happening and it's marginally boring (oh come on, who doesn't like to see Yoon Se Ah throwing a temper tantrum), but I'd do it I swear.
If you're laboring over whether you want to watch this or not, are you looking for something serious? Eh, go watch something else.
Honestly, I enjoy a lot of the cliched plots in melodrama, but it's a rare melodrama that keeps me interested throughout. If you like melodrama, you'll probably enjoy this, and even if you don't, you might... if you get what I'm saying.
Come on, who doesn't love a twins story. It's double the fun!