as madtrang said the main problem is running cable throught the firewall, but its not that bad.
You'll need to make up some MDF mounting plates for the fronts. Just use the stock ones as a template.
I just ran new wires to all the speakers and bypassed the factory amp.
As for subs, it depends on how much boot space you can afford to lose. I need to fit a set of golf clubs in mine so I've got a small sealed box between the shock towers.
Head unit is pretty easy, just get the right wiring diagram off the net. I had to drill new mounting holes in the factory cradle to make my head unit sit flush with the a/c controls.
Let me know if you need any more help as I just finished my install last weekend.