Scan quality totally depends on your device camera capabilities. The light and the initial size of the document you scan affect the quality as well.
iPod Touch 4th Gen and iPad 2 cameras have 0.9 Mpx, which is not enough to create a legible scan of A4 or letter page. Still you may take a picture of a smaller portion of text and create a good scan using iPod Touch or iPad 2.
You may also increase legibility using Scanner Pro adjustments. For B&W scans 'Grayscale' and 'Document' modes are preferable.