In fact, in its class DJ-1800 doesn't really have any competition.
Some professional Mac DJ applications are much more expensive and they suffer often on "featuritis". Others are more in the "hobby-DJ" class (and also a bit cheaper) and thus lacking some essential features for pro and semi-pro DJs which you can find in DJ-1800.

2.2 is the best version ever. The playback is now very reliable (basically due to QuickTime 7 - not all of the earlier problems were the developer's fault!) and the overall performance has been highly optimized.

MIDI support is a nice addition, especially if you have one of the supported DJ controllers (I don't).
However if you want to control DJ-1800 with non-DJ devices like general MIDI keyboards, there are still a couple things that need to be fixed; I hope this will happen in v3.x. (I'll be posting workarounds in forums.dj1800.com/viewtopic.php?t=438)

I use DJ-1800 on an iBook G3/700, and although it's slightly below the minimal system requirements, v2.2 now runs better than any version before. Of course it runs much better on my Powerbook G4/1.67, but I don't use that machine for deejaying...

And finally, it's also necessary to highlight the excellent support from Dave Addey, the developer... :)