The review by pennyelenabooks is pretty spot on, except I think a 6 for this show is very generous. The plot is very simple but I was definitely interested enough to watch the whole show. The cast all acted well but there were a LOT of unlikable characters - pretty much the only redeemable character was Joo In-Ah and the teen girl who she befriends. The cast is pretty expansive and I got tired of the side stories about half-way through. In-Ah's sister is one of the most horrible characters I've ever seen in a kdrama, and to be honest the way the show ended left a bad taste. There were a lot of things left unresolved or glossed over, and as the other reviewer said, the ML's condition was only used when needed. I agree that I wasn't bored, but that was the only real plus.
Overall if you want to watch something uncomplicated and angsty this is an okay, time-passer of a show.