Delay. This Virtual Amp app is called Delay Effects because there will be delay -- between 1/4 second to 1 second depending on your device. This Amp Emulator app takes the approach of accepting that there will be delay and going with it. In fact, you can select an option to increase the delay so you can more easily play in harmony with yourself.

If you have the right cable, you can plug an electric guitar in, otherwise use an acoustic through the microphone (headphone required though). Feedback: in mobile devices the speaker and microphone are close together, so without a guitar cable or headphones, it will feedback. Also, the feedback detection may be triggered if the volume of the input signal is too loud.