It's quite adequate, seems to have a large selection of languages. Whether it uses Google or not is irrelevant I guess, but that's why I chose it.
Correction
(I originally stated): Unfortunately, I really wish it did entire pages on the fly, instead of what you paste in the box.
The page I was on, for whatever reason wouldn't auto inject the translate bubble, and I really thought the add-on wouldn't work for me at all BUT once you go into options and change it to context menu then it's readily available. So keep that in mind before you decide to drop this add-on which I almost did
/Correction
Great job, Karl. Many thanks.

Lo malo que tiene es que no se puede acomodar el recuadro de traducción en el lugar que uno lo desea poner

It seems good and promising!
Interestingly, you can directly write your own text and do instant translation, a feature not usually found in similar add-ons.
However, the UI need some improvements, most important of them being:
- make the translation window movable (drag & move)
- make translation window sizable or auto-sizable, i.e. to automatically adapt to the content.
- add option to show-up just the translated text and selectively hide all or some of the other elements like buttons, drop-down language lists et.c.

Really like this addon, very handy and easy to use.

Would love to give it 5 stars, except Context Menu option not working properly.
ON options page;

Translator Button Type:
Injected Button
Context Menu

When you choose context menu option, then click save changes, it works perfect, until you restart browser, then it disappears from the context menu.

When you do the same with the Injected Button option, it continues to work perfectly.

Please fix, so i can give it 5 stars