I've spent some quality time with one of the latest offerings from IK Multimedia – the iRig Pads. Please see the video below.
Note: I purposely fired it up and tried to run it ‘out of the box' so readers and viewers could see just how easy or difficult it is to use.
Result: intuitive, easy to use, light weight, easy power options, many configuration options
Retail Price $149.99 which places it in range of comparable products though i can't find others that plug directly in to iOS devices. This makes the iRig Pads stand out.
On the down side (to be fair): for ipad2, or other iDevices that are ‘pre-lightning,' the connector costs an extra $30., need to see more apps developed for use, more clear instructions for IK Multimedia apps.