I agree completely. I've been toying off and on (lately on) with my own language that has a syntax between Perl and Lisp. It has scalars, lists and hashes.

Writing the parser has been quite challenging. Figuring out how to create the parse tree, optimize it (constant folding, etc) has been good for keeping my skills sharp and looking at problems in a different light.

Sure I could have started with a book on compilers and read up on lex and yacc, but solving it from the ground up, in my mind, has taught me a lot more than just following some book. Not to say there is anything wrong with that. When I get it to a more stable state, I'll probably do exactly that. Go back and see what was done right and what things I didn' figure out.