I have successfully upgraded three systems to the latest -Current (every computer i have has been running -Current all the time). The first computer i upgraded was my workstation at the office. The second one was my desktop, and the last one was my laptop. I had no major problem while upgrading my workstation, but i do have some problems with my desktop and laptop.
The problem lies on NVidia driver which should be rebuilt after the XOrg updates. Currently i'm using a custom kernel which i compiled it myself, but the kernel header that is on the system is using Slackware-Current's kernel header, so the the installation failed. I have tried to set SYSOUT and SYSSRC environment variable as told by the error messages, but no luck. In the end, i compiled a new kernel and finally the problem dissappeared.
Besides this driver, nothing major comes in my way. I agree with Pat on this one. KDE is just keep getting better, but not only KDE. So does Slackware
By the way, i'm still waiting for an update to Mozilla Firefox. The latest version has quite some important fixes related to security and it should be delivered today (i hope).