If you are looking for a highly capable, customizable and robust application for handling tasks and calendar on WM, you should definately have a look at Pocket Informant (PI). I am using PI on a HTC HD2 and it is indeed superb.
As far as tasks are concerned, my issue with the native tasks application is that there is no option to group tasks by category, as there is in Outlook. You need to filter each individual category. PI offers this, allowing you to select which categories to show - for example, I display @c, @call, @h, @office, @out (my shopping category). I choose not to display .F and .S/M, which I only need to review on the desktop.
The Tasks application has been a major disappointment of WM O/S for a long time. Calendar has been ok. Pocket Informant takes these two to the next level.
Only drawback is that it is very resource intensive. I had tried it on previous WM devices (HTC K-Jam, HTC Universal) and it would make the device quite slow. With the 1GHz processor of the HD2, this is a thing of the past. Plus, the tasks/calendar application is beautiful on the 4.3" display...
Definately worth the 20$ cost...
http://www.pocketinformant.com/products_info.php?p_id=pocketinformant