I won't spoil the story itself, but just tell you what it adds. You get the Card Tables in Serendipity and after you complete the storyline you get Sazh's Monster crystal and obviously if you played FFXIII then you would figure Sazh is a Synergist.
Considering the Card games themselves one of them being Texas Holdem has a slight bug. When you go ALL IN and you have more chips than your opponent you still are betting all your chips. Something I learned the hard way, when I did it to force my near bust Opponent ALL IN. He ended up winning with his 147 chips and I had a little under 4000. This busted me instantly. So hopefully that gets fixed. The second card game I enjoy a lot more than I thought I would. It requires some interesting strategy and is quite fast paced. So even if I was hoping Triple Triad would make a return I was not disappointed in this interesting clock based game. Here is a screen shot of it here.
Overall the story took me no time at all to get through, but I am a pretty good poker player so that may have had something to do with it. Additionally you can have Sazh in your party and these games forever. I also wish these games were a part of the game when I was going for the fragment for Casino coins won. Doing it with the slots was rather tiresome, so those who haven't gotten it will have a much easier time. So if you are thinking about it I would say it is worth it especially for the monster crystal and the additional story and well you can always play cards when you're bored.