Thanks for the pdf, I'll check it out.
What I was thinking of was the fact (with the impl. in the book) that the cost of dynamic dispatch for towers scaled for every operation as the height of the tower grew, which is much worse than the single indirection OOP languages usually have for virtual member functions.
While reading this book, I also thought about the fact that everything is a linked list. Isn't this horribly inefficient compared to classes with contiguous memory, where you can access any member in constant time? Are the lisp's out there smart enough to cope with this?
Don't get me wrong, so far it looks extremely versatile, but the lack of static typing, contiguous data and so on spook me.