With 4 internal drives I have lots of room to play. I'm also a lazy kind of guy.
After verifying the partition and running permissions repair, I made a second clone of my working Leopard volume and upgraded that to SL. Then I downloaded just a few apps (RBrowser, Cyberduck) for SL compatibility. All other major apps are working great: Office 2008, CS, CS2, CS3, iLife, Final Cut Express, QuickTime (X and 7), Firefox, Google Earth. I'm waiting for a SL update to Garmin's Mac utilities for my nuvi660, and also an updated printer driver for a Xerox Phaser 6120N that I sometimes use.
I was using NTFS-3G/MacFuse in Leopard for write capability to my NTFS Windows partitions. I uninstalled these before doing the SL upgrade, then reinstalled them again once the upgrade went well. They only work in 32-bit mode, but that's fine by me. I was disappointed that SL did not come with built-in NTFS write capability.
My only major peeve is that something got broken with the Epson Stylus Photo R340 that I print to -- a USB printer shared by a machine running Leopard on our home network. I can print to it, but I guess it's a problem of this printer's new SL compatible driver and something flukey in Bonjour that I no longer have access to a lot of the printer's settings (e.g., quality, color, etc) like I did under Leopard. I also can't see the status of the ink cartridges, so the printer's info is not getting back to my machine. But, I'm not alone ... there are hundreds of messages in Apple Discussions from peeps with printer problems after the SL upgrade.
Other than that, my SL upgrade experience was pretty painless. I give it an 8/10.