I used to use kubuntu but now am using BT3 exclusively. There are hundreds of slackware packages available, including everything from office to multimedia.
Even if you can't find a slackware package, you can even convert an rpm package to a slackware package. That's just one of the many things one can do. The possibilities are endless.
BT also has it's own package manager slapt-get (command line), also available is a gui for slapt-get, named gslapt.
If you have some experience with Linux and are quite adventurous, I can definitely recommend Backtrack as an all good all round linux operating system.
I know a some people use BackTrack mainly for pentesting etc. and use (K)ubuntu or something for other things. But to be honest, I don't see the logic behind this as Backtrack is all I need at the moment.
Also remember, Backtrack 3 Final and BT4 Beta will be released in the next month or two