Wunderlist have just released their much awaited update which they have been working on for months.
Wunderlist pro brings a new paid subscription. The main app is free to use, but the pro version allows you to work with others in a team.
The pro versions allows you to delegate tasks, add unlimited subtasks, gives you more backgrounds and the ability to attach files. It costs $5 a month or $50 a year. However I don't know if the additional features are worth the paid subscription, they seem pretty limited.
I've got to say though that the service is beautiful. I've used some other to-do list and collaboration services before, but I don't think any of them are as pretty as Wunderlist.