It's good sometimes, but more often than not it can't find a definition, which doesn't make it a particularly comprehensive dictionary. And if a word is a plural or any kind of variation, it can't find it either, just the simple form of the word, and there's no way to get a definition by highlighting a word without the 's' or whatever on the end, because you have to double click, which naturally highlights the entire word.
There's just too many words it can't find.