I was wondering about getting one of these. but i've seen mixed reviews about them. For instance how they are not as sharp, and lose some quality of the image compared to other lenses.

like i said i've only seen reviews online just wanted to hear something from someone that has used ,or tried these out for more than just a test.
are they worth using? or would a 70-210 with 2x tele work just as well?? Thanks

best regards