Eric is right, for the Mac, DVDSP is the only professionnal dvd authoring software (Sonic Creator and Sonic Fusion are discontinued but still useable, if you can find one used, go for it). Like Eric said, its not totally perfect since it uses a abstraction layer that prevents you to have complete control on dvd specifications, but it makes it easier to use. You can edit a lot of professionnal titles without having any issues, but for more complex projects where you need to have tight control on dvd specifications, you will have to look on something like Sonic’s products. You also have DVD-lab Pro (199$), that is considered by some pros as Scenarist’s jr.
Pete