Except for the Pitch Shift calculator I can't see any benefit from this software. Everything else (and much more) you already have in iTunes, although you might need the free AppleScript app "iTunes-BPM" (aka "BPM Inspector") to count and enter your BPM values in iTunes.
And DJ Library doesn't even import those BPM tags when you add songs to its library. Hm.
Does it actually integrate with any existing DJ app? It comes from the developer of "Disco" though -- which is not really a "hi-fi" DJ app anyway...
The Pitch Shift calculator as a stand-alone application with a BPM counter and iTunes integration and with a floating window option. That might be useful. The rest is quite superfluous.