If you apply to a number of the less prestigious places - not very difficult at all, any child who's outside say the bottom handful of kids in a typical state primary class will usually get in somewhere.
If you focus on the most prestigious places - quite difficult. Would need to be in the top handful of kids in a typical state primary class and prepare well for testing etc.
Generally across the board - has recently been more difficult, as noted above.
Emanuel - much more difficult than it used to be, as noted above.