It really depends on what you want to do.. Of course a computer is going to give you more options, but if you want something external and not computer based, the MRS-4 is pretty good.. I have one and use it often, but if you can spend $300 there's also the Fostex MR-8...

Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.