Otherwise okay, but on a Mac it's still only the second best option.
This is because of the interface forces an extra step upon the user: They have to pick which languages to translate with every single word.
This might make sense if the user wants to regularly translate from several languages. But this is really stupid in cases where the user has specified in the preferences only one language setting, like "DE to EN". Ignoring this the add-on still asks the user to click on "Translate to DE-EN" every time. If the uses specifies only one language pair, it would make sense for the add-on to provide instant translations without extra clicks.
On a Mac (I have no idea about PCs) the user can use the build in Dictionary function. With this the user can simply perform a single gesture, for example a three-finger-click (this can be customised to suit ones preferences), to bring up a translation for any word. Nothing extra is required. So at the moment that's the fastest and most user-friendly way to translate words on any web page.
I'm a big fan of using dict.cc, so I would love to see this add-on get better. But until this add-on achieves a similar ease of use to what OS X offers as a default, it simply isn't worth using if you're on a Mac.
First of all: You only need to pick the language if you use the context-menu. You can instead use the "quick click" feature to translate with the top entry in your language list. You can configure it to be activated using a key-combo + click. Alternatively you can use a rocker gesture (tho I guess since macs only have one mouse-button, this is no option for you). I don't think I have a way to check for gestures like 3 finger click.