I use Sonic Scenarist. It was the first "real" authoring program I messed with and I've been able to do pretty much anything I want with it. Of course there's a bit of a learning curve, but once you get the hang of it and understand the structure of a DVD, it's easy to use. I've heard great things about other authoring programs out there, like DVD Lab Pro and Maestro, but I've never tried them myself.