Yes, in macro work is extenders can be useful. One thing I found handy about using an extender is that your subject is bigger in camera. Especially with TS-E lenses where it is hard to focus manually.
Most opinions I heard about the 1.4x were positive. Using the 2x-Extender under good conditions really allows to extract more detail from the subject that it would have been possible without extender. This is especially true when using current FF-cameras. Look at the second image here where the flower is a) cropped-magnified and b) photographed with an 1.4x/2x extender:http://www.the-digital-picture.com/Reviews/Canon-Extender-EF-1.4x-III-Review.aspx
One downside by using an extender I found is that the whole image quality degrades i.e. lower contrast, CA, distortion etc. You have to use postprocessing to achieve the quality of the naked lens.
You should also consider using a crop camera. Especially the predicted 7D-II with 24MP already provides a much higher resolution (in pixels per mm) than the full-frame-cameras. That is like a 18MP full-Frame with a 2x-Extender built in.