While searching for a place to record all of my todos (a la Getting Things Done) I have been through various online and offline systems.
First Joey setup Tracks for me. This followed GTD pretty well, but the UI wasn't really appealing. You also couldn't add repeating events and it took a lot of clicks to manage your todos. You have to install the app yourself and that means maintenance and no new features without work.
Next came Gubb. It had a lot of pretty colors and instant UI appeal but it lacked features. And after awhile the UI that looked flashy at first got pretty clunky. Very slow feature iteration.
Along comes Todoist into the market and finally something sticks. It has extremely quick development, can be used almost entirely by keyboard, looks clean with a very nice UI. Has subprojects, repeating events, very nice search functionality, mobile UI, an api, igoogle gadget and much more. If you want something to help manage all of your tasks this is definitely the one I recommend.