I got three days testing with the SX springs in my '99EXC. The riding consisted of everything from heavenly 4/5th gear single track to 1/2nd gear rocky areas. I used the stock 10cm [I think] preload spacers.
For any trail where I could use 2nd gear and higher, I really liked the stiffer springs. The slow going rocky sections were rougher, but there was very little deflection. It's almost as though I could feel the rocks more, but they didn't disrupt the bike. While experimenting with the clickers, I ended the weekend full soft on compression and three from all the way in on the rebound side. The compression changes really didn't seem to make as much difference as with my stock springs.
The balance of the bike is changed, so I had to adapt to that, plus I decreased the rebound damping in the rear one click to compliment the stiffer front end, which I didn't want to have to do. If I was really going to get serious about making changes to my suspension [the stock stuff is fine by me, I just got the fork springs cheap, so I decided to experiment], I would get the stiffer shock spring also.
For now I am going to leave the 4.2s in and ride a few different areas before making further conclusions. Actually, I want to like these stiffer springs for all conditions because I really like the confidence the stiffer springs provide on fast downhills or G-outs etc.
So, the 4.2s aren't the perfect setup for me, but I may stick with them despite the tradeoff's. Maybe I will try less preload, see if that softens them just a bit.