I already owned a DrumKat before I bought this Alesis unit.  The DrumKat works great, but I needed a unit that I could add on-board samples to, and that wouldn't take up a very large footprint in any of my set ups.  That's where the SamplePad 4 comes in.  Very simple design with 4 playing surfaces, built in sounds, and the ability to add your own samples on an SD card.  Unfortunately, the samples don't actually "load in" the machine-they stay on the SD card, so the access to those samples takes a bit longer than I would like it to.  I think Alesis put this thing together with cheap springs and crazy glue, but it does the job....