I've had a lot of fun using Byteman to debug some HSQLDB code last week.
I wrote a quick tutorial to explain how it works:
http://dev.xwiki.org/xwiki/bin/preview/Community/Debugging#HUsingByteman
Enjoy!
-Vincent
PS: It's really really powerful and simpler to use than Aspects IMO when your use case
is to debug stuff.