This started out really strong and I loved the banter between the main characters, but I feel like it got a bit tired in the middle. It's one of those super frustrating shows, because I really liked the characters and acting, and there was a great storyline...but I feel like it had so much potential that just wasn't delivered.
I did appreciate the switch around with Gong Shim chasing Dan Tae, but I feel like the secret keeping went on for too long and it just got painful to watch Gong Shim continuing to be 'oblivious' to how shut off Dan Tae was being. He realised how his behaviour was impacting her when she got so sick just because she didn't want to miss out on the breadcrumb 'date' (and trying to stay in hospital afterwards to spend time together?!) - but he still didn't then 'fess up.
I do understand why he couldn't let his cousin know though, given how he was acting like a prat to protect his mother.
Plus, I really feel the stuck-up sister was under-utilised. She had great promise as a manipulative character, but that just kind of fizzled out. The end kind of alluded to her feelings becoming real, but there was no big emotional moment, nor was there any confrontation between the sisters, or acknowledgement of her mistreatment of Gong Shim. I kept waiting for it to either be revealed that she'd been swapping Gong Shim's hair ointment and caused the bald patch out of jealousy...or for Gong Shim to stand up for herself and call her sister out for never supporting her and always being embarrassed about her. But neither happened.
I would probably still recommend watching the show. The initial love triangle is really cute and fun, and I loved how the rich/handsome/out-of-her-league Chaebol heir fell for the dowdy poor girl, only for her to then turn him down to focus on herself! There are also several cute side bits (the two husbands are funny together; the grandma secretly vetting her grandson's girlfriend).