It's a relatively simple feature, but could prove really useful for those who want way to get organized without leaving Gmail. You can add tasks and reorder them, and of course, cross out the completed ones. You can also convert emails into tasks, in a similar way to Apples Mail app on Mac OSX or Outlook on Windows machines.
There are still some important features missing. For instance, you cant share tasks with others and the service lacks integration with Google Calendar. Still, its simple, its free, its portable and it works. As a side note, if Gmail Tasks is too lightweight for your needs, then you might want to check out Remember the Milk which also works within Gmail and supports offline usage through Google Gears.