very very very easy, dash pops right off, two phillips head screws and two 10mm bolts and the factory HU comes right out... the kit I have is a cheap one but it fits nicely, just don't torque the 10mm bolts down too tightly or it breaks :-)
Whew... Got my new head unit installed as well as the steering control module, thanks to the help of Electrodynamic. Looks good but the installation of the steering control module (PAC SWI-ALP) was a little more involved than I anticipated. Still very happy will post pics soon!! PS- Sounds really good.
Welcome to the forum andersnor, you have made a good choice, I have only been on the forum for like 3 days, but you'll love SI's. But, only if you like, clean - hard hitting bass.