RadiantQ’s Silverlight Gantt works well, and they’ve been helpful so far by providing useful samples when requested. When a bug has been spotted, they’ve been quick to provide a fix.
We had started to use IBM ILOG Silverlight Gantt in our product, but IBM’s support was very poor, and the component was really locked down, and because there was no source code we struggled to get it to do what we wanted.
RadiantQ’s Gantt is much more flexible though, and we purchased the Source Code from them, so we can see exactly how it works.
So far so good!