I’m really digging the Souuus i2M Musicport. It is pricy ($150) compared to some USB guitar inputs. That is, until you consider the added functionality. Not only is it a clean analogue audio input (for guitar or bass), but it also does fast tone detection onboard to output MIDI. The MIDI is monophonic, so don’t expect it to do chords or individual string detection like a Roland GR-55 setup, but for synth leads or sax solos it’s awesome.
The i2M comes with software for customizing the device to your needs, but operates independently of software. So you can use it on a computer as well as an iPad. Makes a perfect choice for iPad Garage Band since you can feed the guitar amps the analog signal and send MIDI to the keyboards, drums and other instruments. Coolest feature – the SONUUS logo lights up (green or red) and flashes to indicate mode and clipping.