For more complicated authoring, I've used DVD-lab PRO, which is pretty easy to use and quite powerful. However I did once have a problem with it not recognising a text file containing timecodes of desired chapter stops.
Scenarist is, of course, the industry standard. I've never used it - the version I had, when I tried to install, turned out to be a trojan (bloody Suprnova
). Apparently it's very powerful, but as Zion said it probably takes some time to get used to.