Where to Start?

[cmims]

Sorry, found some of the answers after I first posted and could not delete, so I have edited to focus questions. I would like more opinions on the Lego NXT system. Is it challenging enough to really learn on? Is there an after market if you want to moved up to more advanced robots? Concern is the toy like nature of it, but the fact it works with MAC native and has a low entry cost makes it interesting.

What do you need beside basic kit to really get going?