I hate to admit it, but I am more productive when I have a task list to keep me moving forward. I've used, over the years, tons of tools (both paper and electronic) to help me with this, and I've found that having an easily portable system is crucial.
Because I'm lazy, I also want something that integrates with the rest of what I do so that I'm not scattered in 93 different directions when I try to remember where I wrote something.
Google Tasks covers most of what I need from an electronic task list, and GeeTasks is a fine way of keeping my Google Tasks in front of me when I'm moving around in the world.
With GeeTasks I can look at tasks created on my computer, mark them complete, and create new tasks with due dates. If I have multiple task lists (for the multiple roles I play), GeeTasks can show all of them and allow me to easily work among them.
If I'm away from a connection to the Internet (say, on an airplane), then GeeTasks will still allow me to make changes to the tasks on my lists and then sync those changes when I'm next connected to the Net.
In all, the limitations of GeeTasks are the same ones inherent in Google Tasks. If you're willing to live with those, then GeeTasks is a very good way of keeping me on task and on target.