Has anyone tried this extender yet? I am looking at this Canon 2x version II Extender to be use with my 70-200 f/2.8 IS and either of the following:

Canon 300mm f/4 or the Canon 100-40mm f/4.5-5.6

If anyone has tested this piece of glass I would be thankful to let me know their feedback.

PS I already use the Canon 1.4 Extender II.