In my experience...
I love how simple Slackware feels (it feels like the BeOS of the Linuxes, in a way :). You of course need to learn 5-6 Slackware-specific tricks and things, but your adventure should stop right there. Everything should just work, manipulating services, changing configurations, etc.
Installing applications is very easy, you just download .tgz files from either the Slackware FTP site or the third party apps found at LinuxPackages.net and then you type "installpkg file-name.tgz" or "upgradepkg file-name.tgz" or "remove app-name" and the rest is history. The funny thing is that while Slackware's tools don't support dependency checking (third party package tool Swaret does) I did not usually have any problems installing anything. The packagers make sure that all dependencies needed are to be found on their web site or on LinuxPackages' site usually grouped together and they are easy to find these groups, and so it makes software installation pretty trivial.
To demonstrate how much the Slackware guys and its developer community value backwards compatibility for the convienience of the user, downloading and installing packages and its dependancies manually from the Dropline Gnome distribution it works perfectly with stock X11 or stock Gnome (for example, I got the xine, totem, Evolution packages from Dropline and they worked fine on my stock X/Gnome even if Dropline offers its own version of X and Gnome.) The only problem I hit sometimes when I install gnome packages is that scrollkeeper takes forever to do its part. This is a known behavior that happens in a number of distros and in my opinion scrollkeeper should be fixed or re-architected.
Creating packages and sharing them with other Slackware users is easy. Instead of the usual "make install" after compiling something, just do a "checkinstall -S" and it will create and install the .tgz package for you! The .tgz file is now ready for sharing with others (tip: on Slackware is best to always configure stuff as ./configure --prefix=/usr because some specific libs don't like the default /usr/local location, even if it's on the $PATH).
The other great thing about Slackware is that its -CURRENT tree, the unstable one, is actually... very stable. I admit that I am constantly running -Current on my desktop (not /testing though, which is really beta) and I haven't noticed any problems. Things continue to work as they did before.
Speed-wise I consider the 486-compiled Slackware the fastest distro on the machine that it shares with i686-compiled Fedora, Java Desktop System, Lindows, SuSE and Mandrake (along with other OSes too). Slackware is the snapiest of all under normal Gnome desktop usage at least. And it boots much faster than any of the other distros too. About 20 seconds boot time on this Samsung 40 GB drive (with DMA on).
In my review in September I mentioned a problem with the software mixer found on the VIA AC97 on-board card, but installing my Yamaha XG-754 worked like a charm with ALSA and I am quite happy with the quality (there are some noise artifacts sometimes in some alert sounds though).
Slackware has been rock solid for me. I know of two reproducible ways of crashing its Nautilus and one of crashing the included KOffice, but that's it. I mean, if you know my reviews, you know I can always find bugs and problems all over the place. Slackware has proven 'more than good enough' stability and bug-wise.
Slackware takes the "safe approach" and it doesn't patch the kernel with custom hacks like some distros do. They only use the default kernel code pretty much. But having said that, it still works with my FireWire PCI card and my new USB camera out of the box. Only the mouse movement is pretty bad on X, but all distros have that (except Fedora as it has special kernel patches for it).
By now you probably have noticed that my two Slackware articles I have ever written (this one and my Sept. review) are pretty "short" for my standards. This is because there is nothing either glorious or problematic to report. It just works. It is a distro which doesn't have to "pretend" about anything. It is what it is and does nothing more or less than it markets: a Linux distro based on the BSD layout which makes a good small/medium business server and if you are a more-than-average user should also make a good desktop too. It promises nothing more and it succeeds to this goal as it doesn't have many of the annoyances found elsewhere.
I think I have used more than 25 Linux distribution versions in the last 4-5 years (more than 12 unique ones). Slackware is the first one that "stuck" on me. Not because it fixes long term problems with the Unix/X usability or doing anything so different from the rest, but because it is simple, it is easy to understand its "way", it's stable, and it's easy to install applications. In general, Slackware wins on the edges, it is not that different from the rest, but different enough on the places that count to offer a consistent experience.
The only thing I want to see from Slackware in the future is more research and effort besides the "package, test, ship" direction. Slackware hasn't changed much the last few years and it already shows its age compared to more modern OSes. Some effort to include integrated server/system tools for various tasks and some differentiation on the desktop side would be most welcome. Oh and Patrick, please add FAM, I can't live without it.