Susan,

Most certainly take a look at OmniFocus. It's been developed from the ground up as a GTD app. The developer, Omni Group, has a proven track record as an excellent Mac app development shop.

I've used OmniFocus since early beta days (it just shipped 1.0) and have been quite happy with the system I've now got in place.

Your mileage may vary; it is a personal decision based on how you best work.

Cheers,

andy