Equipment

the Kontrol S4 No it's not a spelling error, it’s a piece of hardware that replaces the function of the mixer and allows me as a DJ to control my computer program like others control their mixers and CD-players.
the LPD8 Knobs, knobs and ohw yes ... Knobs! One can't have enough. I assigned customized effects to the LPD8's knobs to add a little extra spice to my DJ sets.
a Notebook with Traktor 2 The music is stored on my notebook as MP3-files. I use a computer program, Traktor, to manage all my music files. Traktor also makes a visual representation of the song in which I can indicate where the cool parts are or even loop a cool part so that just that one part is played continuously. Finally it generates and mixes the music, sends it to the Digital Jockey which then produces the actual sound and sends that to your sound system.
Headphone & Microphone The final parts of my equipment are the same as any DJ: a headphone and a microphone.

Pro's & Con's

Price: a Reloop Digital Jockey costs around € 320 and comes with Traktor included. A CD-DJ will have for that same amount of money just one midrange CD-player. And at € 1,30 for an MP3-download it’s cheaper than any other format out there.
I currently have three days of music indexed in Traktor, that’s the equivalent of roughly 60 CD’s.
I can indicate cool parts, set loops, create playlists ... all when mixing live or in advance.
If your power fails, thanks to its battery the notebook will still keep working, allowing me to prepare a come-back as if the black-out was on purpose ...
Unlike CD’s or Vinyl Records, MP3-files stored on a well maintained hard-drive don’t get scratches by being played to often. They will always sound brand new.
If a in internet connection is at hand, I can update my musical collection on the spot.

Computers do crash, but with proper system maintenance and weekly back-ups chances are slim.
Computers have latency, but using external hardware (like Reloop’s Digital Jockey) to generate the sound eliminates the computer’s latency.
Using a computer to DJ is not actually DJ’ing? The Vinyl people said that same thing about CD’s years ago ...