After 2 installs, I've found:
On my macbook, it installed without a hitch - did its' thing, rebooted, came into SL - a few preference panes didn't work anymore due to incompatibility (blueharvest, istat menus) but I was able to either just not use them or delete them. Never saw an incompatible applications folder, and everything appears to be where it should. Looking at activity monitor, most things running 64bit, a few not - mostly processes from vmware and from FCS2
On my Mac Pro - this one didn't quite go as smooth - sat at a blue screen with a cursor for quite a while before I decided to powercycle reboot the machine. It seems to be ok, and checked the disk and permissions and all appears to be running well.
I did install everything, and did notice that it is much smaller then leopard - people seem to say that it's smaller because you don't install all the printer drivers - the printer drivers are only 860 megs worth of the os for all of them (which I did select to have).
I have noticed reduced performance of COD4 (not a huge drop, but I still can see it on my fps counter in game) - I'm wondering if we won't see real huge graphics performance increases until games or software comes out that really supports opencl.
Some apps have felt a little slower to launch the first time on my mac pro (for example itunes the first launch after upgrade took quite a while, but subsequent launches were 1 bounce and felt faster then from 10.5)
I have mixed feelings about the new quicktime. I do have QT7 on my system as well, but the controls on a small window video do take up a lot of space. I'm also a little disappointed that it doesn't support more formats (like .mkv) out of the box, and perian doesn't support quicktime x, so I'll still be using QT7 and VLC for a bunch of my video files.
Overall it doesn't look much different from the outside, but I do appreciate the unseen changes that will enhance the OS overall, and think that even with some of changes rendering some things incompatible, it's a step in the right direction.
Additional Notes:
I have the Brother MFC-6490CW printer, I had to reinstall the software for it to have network scan functionality back, although the base printer drivers still functioned after the update to SL.
Playing video in the finder is cute.