Most of the negative reviews for this seem to be people who don't understand what it is. I'm not really sure how someone mistakes it for a modern English dictionary since it does not say that in the title at all.
This is a useful reference guide for words from the language of Old English, i.e. the language spoken in the early middle ages and not to be confused with Modern English, what we speak today. Understand that Old English barely, if at all, resembles Modern English. It is essentially a completely different language and this is basically a translation dictionary.
That said, even understanding this, I was surprised to find some basic words were not included, such as 'ætheling'. Perhaps the dictionary is assuming that a student of Old English already knows and does not need to look up such a common word but I would have liked it to be complete.
The Kindle edition is searchable and one does sometimes need to use the proper unique characters to get the correct search results, though not always. For example, you can search for 'acalan' and get results for 'ācalan' but you can not get results for 'æthebban' by searching for 'aethebban'. Additionally, it does not have a table of contents so you can't jump to a certain letter.
Though I would like it to be more comprehensive and the search function/navigation could be improved to accommodate for the lack of unique characters, the Kindle edition is available for free. As a freebie, it's fairly useful especially given that I didn't notice any OCR errors that often plague public domain books.