I have finally bitten the bullet and am installing 10.6 over 10.4 on a trusty iMac as I write. Installation says it will take 43 minutes.
(We've got 4 Macs here: 2 came with SL preinstalled, the others were on Tiger, waiting for me to get used to SL and go for it. I'm writing on a new SL Mac, which I've got used to.)
Before I put the instal disk in, I copied over all the iPhoto photos and some other user files.
It WILL come back up with all user files in place. I'm sure it will. I told it to.
more follows when instal finishes.
Exciting!