A long-time user of OTSAV-DJ software, I find Mixxx, Ableton Live, Traktor DJ, and Serato looking most attractive. All four companies offer great products.
In 2012, the user interface was totally changed with version 1.90 of the software. Three years later, I am still uncomfortable with the new.
A key element of using software for live DJ sets is its link, known as midi mapping, between the hardware control surface and the computer it runs on. Keyboard and mouse alone are often inadequate, and there is a large, fast-developing choice of controllers available. Sadly, OTSAV provdes mappings for a very limited set of devices, although most others can be used if one is prepared to do some non-trivial programming. For radio work, it's still a highly competent product.
It only runs on a Windows PC, so has its limits.
Although the PC and Mac offering from Native Instruments, Traktor Pro 2, is very popular and a competent DJ product, I agree with those who find its interface cluttered and the program complex, tricky to set up and configure. Frequent referral to the manual seems essential.
On the other hand, Traktor DJ running on iPad and iPhone iOS devices makes good use of touch-screen functionality and is remarkably straightforward to use. It quickly became a must-have for review of new tracks in multiple situations and it makes a good DJ backup solution with the addition of Apple's 'Composite AV Cable'.
Like Traktor Pro, the on-screen appearance of Mixxx is cluttered, but it has significant redeeming features which will make it a winner for many.
For starters, its software is free for inspection or modification, free to download, and it runs on all three main computer platforms: Ubuntu GNU/Linux, Windows and of course, the Apple Mac's OSX.
Mixxx has iTunes Integration, so with a good broadband connection, playlists and songs can be made ready to go for a live DJ performance.
As its name suggests, mixing functionality is second to none, with BPM tempo detection and sync, auto DJ and its out-of-the-box support for over 30 hardware midi controllers. It supports multiple audio file formats including MP3, M4A/AAC, OGG and FLAC as well as timecode control and Shoutcast broadcasting.
Definitely one to keep!
The interface of Serato between its human users and the underlying technology is uncluttered and relatively intuitive whilst providing impressive functionality. Key to this is its close tie to specific hardware, allowing mutually beneficial parallel development and licensing. In fact, Serato will not run as DJ software without the computer being connected to a mixer, midi controller or sound card from the list of supported devices. On the plus side, Serato DJ is free to download and run with these devices.
Serato is almost a de facto DJ industry standard which runs on de facto music industry standard computers, the Apple Mac series.
Ableton Live is a music industry leader for producers and performers, including DJs. In combination with a Mac computer and one or two midi controllers, one has a truly outstanding set of tools. (It also runs on a Windows PC, but I would not trust the latter in a critical situation.)
It has two principal views onto music: the arrangement view shared by nearly all its competitors, and the session view, making it akin to a modern DJ software suite.
This will probably become my main attention grabber, perhaps eclipsing the others here.
Yes, I care a lot about music software. As with many areas of life, it's 'horses for courses', and all of the above have a place in mine. For now, at least.