I have a 2001 S. The replacement was very easy, less than 30 min.
I got Kenwood 1677's for the front 2 years ago. They made a huge difference in the quality and volume. They were only about $20 on ebay. They fit perfectly without any kits or adjustments.
I changed the rears last year to Kenwood 1068S's. They did improve the sound a bit but not like the fronts.
gerry