Ubisoft has revealed there are no plans to release a demo for Watch Dogs.
Watch Dogs’ creative director Jonathan Morin commented on twitter on the topic:
Doing Demos take time, the focus is on the game. You will see more footage before release. Be patient & thanks for your passion. :)
He confirmed again in another reply that there will be no demo released for Watch Dogs comes out on May 27th.
If you’re still on the fence whether you want to buy the game or not, renting is your only option. It appears a playable version of the game doesn’t seem like it will happen until the game is 100% finished and ready to ship.