That looks really nice. And I like that it defaults to doing the right thing. I always have to read the docs when using gdb, as it happens so rarely that I forget the commands for doing things...and it definitely does not default to useful behaviors, certainly not just hitting "enter" to do the next obvious thing.
A debugger doesn't take the place of a good REPL, but it's a useful component of a good REPL.
There is a lot of shifting around on the #perl6 chat room in recent months to get Perl6 interacting with Jupyter aka the iPython notebook. Which means you'd probably be able to use iPython command line as a command line client, just without all the Python bells and whistles. If you're interested in seeing that through I'd make some noise in the chat room. Even just saying you'd want it will go a long way! Timotimo looks to be the person to hi5 as he already has some code https://github.com/timo/iperl6kernel
Competent Jupyter integration would be amazing. I haven't had reason to work seriously in Python in many years, but iPython is just fantastic, so having that kind of capability for Perl 6 would make Perl 6 even more attractive for me. I think a lot of folks just don't realize how cool it is, until they actually work with it for a while.
Not necessarily just from a debugging perspective (though I did mention debugging, specifically, above), but from an exploratory programming perspective. The ability to start with a snippet of something, and then trying new things. I often don't know how to solve something until I've tried several ways of doing it. A REPL (a really functional one like iPython, or, I imagine the better Lisp or Smalltalk REPLs, though I've rarely used either language) is a really nice way of doing that.
A debugger, in my mind, is for running the program as it exists on disk and then inspecting it. A REPL is for interactively writing a new program. It is a different way of thinking about programming. On one hand it's something you do first in your editor and then you fix the bugs in the debugger. On the other hand, it's something you tinker with until you're happy and then you commit it to disk. Even when I worked in Python, I would mostly write the code in vim, but when I was trying to understand a new library or sort out the right data structure or algorithm for something I'd use the REPL.
I guess the lines are blurry. iPython is called a Python shell, or was when I was using it. So, saying "REPL" may be misleading, if your vision of a REPL is merely "it executes a statement". You can build up quite complex functions in iPython and execute them on arbitrary data. It doesn't restrict you to one-liners.
If the Perl 6 debugger allows defining new functions (and other stuff beyond merely tweaking data), then it's probably in a similar category, if still immature.
A debugger doesn't take the place of a good REPL, but it's a useful component of a good REPL.