You can buy "real" commercial dictionaries, too. If they are "decent" enough I can't judge, but you can see yourself:
https://store.obreey.com/eng/dictionaries/
It is a great feature that you can build and add your own dictionaries. One point is that - let's take the Sony as an example - the built-in dictionaries are all English focused and cover only a very small part of possible translation pairs. You don't have an English-Polish dictionary, you won't find a German-Portuguese dictionary and you can't add one if you would find one anywhere.
On the other side, many contributers have helped create the online dictionaries on this site:
http://www.dict.cc/
And thanks to the open dictionary feature of the Pocketbooks we can use these dictionaries on our readers.