I used to debug quite well in 8.0M1. Since then, I've tried 8890 and 8940 and experienced the following problem when a breakpoint is hit:
The stack trace pane takes a very log time to refresh. You can watch it render each line in the stack trace from top to bottom taking several seconds per line. On a very deep stack frame it can take minutes to complete. Until it completes refreshing you cannot continuing stepping into the code.
I am running a MacBook Pro, 2.4Ghz Intel Core 2 duo with 4GB RAM. OS X 10.5.5.
I can revert back to 8.0M1 and the problem does not occur.