I am going to share my small experiment comparing FineScanner Pro, knfbReader and TextGrabber, as I currently consider these iOS apps the best choices depending on what you mostly want to use text recognition for. Because OCR apps differ in price greatly, this comparison might help you to make the right choice for you.
I took a photo of an opened book with the standard Camera app, that means without any software guidance tools, such as those in knfbReader, nor with any human help.. Then I loaded the photo to each of these appss, run OCR on the image and compared the results so that the same image is processed in all the apps.
TextGrabber had precision which I would approximate to 35%, although in better conditions when I scanned a flat, clear and well lit document, the precision was almost 100%, specifically, the only error was in some words near the bottom edge of the document where the higher shot angle is starting to affect the readability. FineScanner turned out to have the best results, with precision about 98% compared to about 90% of knfbReader. Though it's worth noting that FineScanner uses online OCr and the processing takes much longer, about 40 seconds in my case but it may differ, compared to about 3 seconds of offline OCR of TextGrabber and knfbReader.
Nevertheless, FineScanner may be preferred since, as the only one, is able to process more images at a time when Images are selected from the camera roll, though Braigo Companion, another OCR app which is free of charge, can do that too. KnfbReader have a batch mode to capture more photos and process them at once, but cannot do this with pictures from the camera roll. Another advantage of FineScanner is that it is able to also recognize structured and formatted text like headings, lists and tables, and save such rich text to formats like Microsoft Word or PDF.
The bottomline, at least from my experience, which may differ from your conditions, is that if you want the best OCR and don't mind a higher cost, then knfbReader is the right choice and is the one which is the most easy to use and the fastest to retrieve the text which can also be easily read back and navigated within. But if you just want to discern what is a paper or grocery package in your hands about, or what's roughly going on your computer screen when VoiceOver is not reading en error message, then TextGrabber is a sufficient option. However, as of now, I cannot much recommend FineScanner due to the accessibility related issues mentioned in its AppleVis App directory entry, but our kind requests to email@example.com may one day result in an affordable good OCR app which would combine the advantages of offline OCR of accessible TextGrabber with the best OCR precision of FineScanner, both products of the ABBYY company which provides its leading OCR technology also for knfbReader and well-known FineReader for Mac and PC.