For good ergonomics while sitting, usually you want to have the laptop up higher on a platform so the screen is closer to the height of your eyes when you sit up straight with your arms at desk level. Which means the laptop keyboard and trackpad are not in a good position for your hands. Many people use a separate keyboard and mouse so their hands are in a comfortable position on the desk while the laptop screen is higher up.
Apple now has the separate Trackpad for this reason if you prefer to use a trackpad over a mouse. Depending on what kind of work you do some tasks may be easier with a mouse.
Personally, I think it's easier to do accurate dragging, clicking and moving at the same time, with a mouse rather than a trackpad - like selecting text, and selecting graphics in photoshop.
If you have OS X Lion, there are some gestures designed just for trackpads.