I've also used Slackware for years...for a time between '98 and '06, Slackware Linux was the server OS of choice in my shop. Like others here, I never really upgraded those systems between versions...there was never a need, other than adding the occasional security patch.
I just did a fresh install of 12.1 on a laptop partition. I've been using the Ubuntus for my day-to-day stuff for a while now, but I missed the get-your-hands-dirty enjoyment of booting for the first time and hitting that familiar terminal prompt. I also enjoyed seeing that the system initialization file structure hasn't changed much from the old BSD-style rc files. I jumped right back into hand configuration once again.
A couple of folks have complained about the KDE default GUI. Xfce is also installed and is easily available as a choice, once which I also prefer. The standard install adds a number of KDE apps and tools to the xfce menus, also a nice touch if you like some of them (I prefer konversation for IRC myself, and k3b for CD/DVD burning).
I also spent a few hours building my own kernel, something I've always done on my servers. Having a kitchen-sink kernel for setup is fine, but I believe small is better, so I like to strip as much unnecessary stuff out as I can, and build modules only for my existing hardware. This makes booting a bit faster and keeps the bloat out of the kernel. (I recall when the kernel build would warn you if the kernel image was too big to fit on a floppy drive. Don't see that warning anymore!). One bit of advice for those considering rolling their own kernel: unless you build an initialization ramdisk image (initrd), make sure you compile your drive's file system support (ext2, etc3, reiserfs, etc.)into the kernel, and not as a module. Things won't boot if you don't!
One other recent issue I discovered is one that some might have experienced if they use Grub as a boot loader vice the Slackware default LILO. I installed grub in a separate partition on my laptop disk some time ago. I wanted to have full control over the configuration, rather then let things be arbitrarily changed or "updated" by other system installs. The problem is that the version of grub 0.97 I installed wouldn't boot my new Slackware 12.1 installation. I could boot via the DVD, and could even mount the Slackware partition after booting Kubuntu or Xubuntu on other partitions.
Turns out that the default inode size for ext2 and ext3 in Slackware 12 is 256 bytes. Grub, by default, looks for a 128-byte inode on the file system, and when it can't find it, it fails, usually with a grub error 2. On the Slackware DVD, you'll find a version of grub that works under the "extras" directory. If you install that over your current grub boot files, things should work fine. I won't go into the in-and-out of a grub installation, since that varies a lot based on how you set it up. I fought with this for a couple of days before finding the reason for the problem on line.
Please note that this isn't a "bug." Slackware has always used LILO as the default boot manager. You could also change the inode size on your system disk when installing Slackware. But if you install the system out-of-the-box and use the tools provided, you'll have no issues.
I'm quickly getting things set up my way, and really enjoying using Slack for mt everyday system again. Old can be beautiful.