If you've managed to miss any of Ubisoft's previous giveaways, you're in luck as they have just brought all three back! If you're interested in a free copy of Assassin's Creed: Black Flag you should head over here, while you should go over there for World in Conflict, and over yonder for Watch Dogs.
It is important to mention, however, that this promotion will only last until 10am UTC on December 23rd! On the positive side, you don't actually need to install the games themselves in order to unlock them for your account. Simply add the CD keys to your Uplay account and you'll be good to go.
As for whether the games are worth your precious time, I would say the answer is a definite yes. World in Conflict is an impressive RTS with a highly enjoyable singleplayer campaign, Assassin's Creed: Black Flag is one of the best games in the entire series, and Watch Dogs, while somewhat mediocre in my eyes, is still well worth checking out. So for the low, low price of free, I honestly don't think you can go wrong with any of these games. Have fun!