Things I love about it:
• Even at its highest quality, it colors the sound of anything you sample into it.
• The built in effects are actually quite good (especially the very unique distortions) and you can process external signals through them by using the mic/line input normally used for sampling.
• You can knock the sample rate down to obscenely low levels for the grainiest sounds you've ever heard.
• It has some weird quirks in the OS that can be fun to exploit. For instance, if you set a loop point too short, it will play back through its entire memory from start to finish. Depending on how many instruments you have loaded, this can give you a sound like a wavetable synth puking its guts out.
• A third party company called Waveboy made some outstanding loadable effects for it including a killer resonant filter (the 16+ has no resonance on its filter), a nasty FM effect, and a cool vocoder. All are GREAT for experimenting and making weird, new sounds.
• It's cheap as hell