It's not as easy as setting breakpoints in an IDE like Eclipse and using a visual debugger there, but it's definitely a workable command-line debugger. You just need a little patience in order learn how to use it, and also probably have a pretty good understanding of Scheme's execution model, but that would be true for debugging any programming language.
Sorry for my tone, I guess YMMV... I just never had any issue with MIT Scheme's debugger, although I admit that CL with SLIME is a bit more useful, especially compared to MIT's weird Emacs-clone (or -fork?) "Edwin".
It's not as easy as setting breakpoints in an IDE like Eclipse and using a visual debugger there, but it's definitely a workable command-line debugger. You just need a little patience in order learn how to use it, and also probably have a pretty good understanding of Scheme's execution model, but that would be true for debugging any programming language.