Quick Take: Some unique options for people who play out frequently, but with some puzzling limitations.
Two things to know right off the bat about Rekordbox: It's released by Pioneer, and it's free. It's a brilliant move if you reckon by Apple's standards, because Rekordbox is set to supplant iTunes and consumer-grade software.
Whether or not this is a good thing depends upon how often you use CDJs. If you're a top DJ on Planet Earth - well, first, thanks for reading! But secondly, the DJ Gods of the World have technical riders and can demand certain technical requirements of the equipment a venue uses. If you're a somewhat lesser deity, then you don't. And though Rekordbox has some decent flexibility, it still won't be a very good solution for you if you're not using a supported pair of turntables (see below).
The metaphor here is not much different than iTunes - player at the top, music library at the bottom - but with several DJ-centric options, such as setting cue points, viewing waveforms and a scan that works similar to the "needle search" on new CDJs.
DJs badly need some kind of specialized playback like this, but Rekordbox is decidedly not for everyone. Several CD turntable units are not supported. Show up for a gig with your tunes all properly cued up via Rekordbox and you might be in for a nasty surprise. This calls into question whether you want to put in the considerable outlay of time to tag your library with Rekordbox.
DJs are, by nature, audiophiles (let's ignore for a moment the ones ripping tunes from YouTube at 128kps). If you're going to make a DJ-caliber iTunes, this has to be taken into account. And a notable compromise for audiophile DJs who have gone digital is to use AIF files, which pack more punch than MP3s and, unlike WAV files, can be tagged with track information. For reasons I'm sure have more to do with obscure licenses that nobody but encoding geeks know about, these are unsupported by Rekordbox. It's a rather obscure limitation but something that will probably frighten off DJs who take audio quality seriously.
Rekordbox is very greedy with system resources, but you should certainly give this a test drive - the price tag of $0 and ubiquity of CDJs means this is quickly becoming an industry standard, if not the only one.
Where to Get It (Worldwide): Rekordbox is free, in the sense that you won't have to pay money for it. You do however need a "License Key", which is silly because you can also get one free of charge from Pioneer. Everything you need is at prodjnet.com.
Technical Specs: Compatible with the CDJ-2000, CDJ-900, CDJ-850, CDJ-350 and DJM-2000. For your computer, requires a Mac running OSX 10.4.6 or later with 1.83 Intel Core Duo or greater processer, 1GB+ of RAM. For Windows, Rekordbox is supported by XP (but not XP Professional x64), Vista and Windows 7, PC/AT compatible with 2 ghz or greater processor. And either system requires either a USB storage device to transfer your music to, or a LAN port if you're connecting from a laptop or desktop directly to a CDJ.
And note the disclaimer: Full functionality is not guaranteed with all computers even when the above operating environment conditions are fulfilled. The truth is that like iTunes, Songbird and damn near every digital music player in existence, Rekordbox is far more bloated than any modern music player needs to be. You can say that it's probably due to the display of waveforms and other cool features rather than holding video capability or something else iTunes stuffs in the box that you're not really interested in, but it's still true that Rekordbox is a system hog.
