I have discovered a number of peculiarities with the DIGITAL debugger, ladebug. However it is still a very useful tool, and some of the problems are likely from my lack of familiarity with it. However, just to pass on my experience...
Prbgrams should be compiled and linked with -g -ladebug.
The GUI version is invoked with dxladebug -iow. The iow switch creates an interactive window for program io - in particular for typing in mofia commands. I have not succeeded in entering mofia commands without this window. However, the program crashes when invoked with -iow from an NCD terminal, though it's OK if you telnet from a linux box or other alpha. I usually use the non-GUI interface.
The program displays local variables which are pointers or user-defined types correctly. Variables defined in a module (above the CONTAINS) are supposed to be visible by prepending $modulename$ to the variable name, but this does not seem to work. Using different case combinations doesn't fix it.
Mostly the coordination between the program action and the listing file is OK, but I have seen them occasionally off by one or two statements.