DDT has a rapidly growing userbase. Here are some of the things they've been saying about it.
Intuitive and clean, the DDT debugger is much easier to learn than its competitor. The real selling points for us were the energetic and knowledgable technical support, which has been consistently over-the-top fantastic, and the reasonable pricing, enabling us to afford a many-processor license.
University of Chicago
A code that employed non-blocking sends to overlap communication and computation was showing extremely poor scaling on a shared memory Sun platform. While stepping through the code with DDT, it was possible to observe that a receiving process stalled inside MPI_RECV until the sending process also called MPI_RECV. This stalling did not occur when blocking sends were used and the code with blocking sends scaled well. DDT guided us to where the performance problem lay much faster than traditional "printf" debugging could have done.
University of Liverpool