I've done it many times there's a really good place nearby. 
I would go once a week if I could . You can get into a very deep 
space doing this but I've found it takes a little practice . You realize 
pretty quickly how cluttered and noisy the mind is and how it can be 
difficult to  quiet your thoughts at times . I try to go for the longest session 
I can because it takes me a while to relax and calm  my thoughts down. 
I think it's the deepest you can without any drugs in fact in my opinion 
It doesn't really need anything added to it . It  really stands alone and has huge 
potential for exploring the mind and actually making positive lasting changes in your life.
The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
Arthur C. Clarke   
http://vimeo.com/32001208