Agenda at Once is definitely a strong competitor in this field. The base should be integration of ToDo with calendar, PIM contacts is secondary (e.g Time & Chaos has fine PIM, weak To-Do).
Nobody is perfect, I found that out quickly, although ToDoList (AbstractSpoon) is nice and MLO, with some calendar lacks and weaknesses if I recall right. And Priorganizer too, although it has no blog entries since Sept 2006. Quick ToDo was pretty, but lacks hierarchal tasks.
One basic is sub-tasks, and Agenda has that nicely. Similarly recurring tasks. And the ToDo is very nice in a couple of ways, easy to move the view fields around, and both a Category and an Assignee (you tend to want to have multiple groupings, I would even like one more, user defined.. e.g. "where" so you could group together by another conceptual basis).The Notes implementation related to Tasks is simple and clean. Thinks like priority and sorting by various ways looks very nice, I think that is partly a function of solid programmers seeking out good toolkits and mixing them together (that is also the difficulty of this whole genre, nobody can easily pick up these programs if they are ever put down, as occured with Keynote. However if the program is good enough, then you don't mind so much, and you hope it will be lasting).
As mentioned above there could be better ToDo-Calendar integration. Time & Chaos had a wonderful drag and drop of a contact into the scheduler. Stuff like that is really kewl. If feels so efficient, they would even bring the phone# in !
And the contacts in Agenda is a bit shallow, it looks like a more recent add-on, and needs the type of field flexibility that the To-Do list has in viewing. Anyway it is nice that there is something there to build on, some good ToDos have nothing.
I don't think you can move the tree of To-Do around, e.g. - make a task into a sub-task. That becomes real helpful if you working with your structure and some of the ToDo programs have that nicely (probably Abstract Spoon and MLO).
One area I have not tested yet is printing.
Anyway, overall I'm impressed. Despite my harping on some needs and weaknesses, it is strong enough to be a keeper at this time. And give it real use time (bought under Bits). It seems like the developer is working hard on improvement. Very good, and I do not know another program that is better on the overall view. Hopefully they can implement tree/subtask manipulation and better integration. If that is in the works in the months ahead, then all is fine and the birds are chirping.