I have the Bushnell 2 1/2 Elite extender and if it fits the binocular good it is a tool to use,it will not replace a spotter of course. I know of one person that uses this same extender on his Nikon 82 Fieldscope when using the lower power eyepiece to up the power to be able to confirm what he is looking at.
http://www.birdforum.net/showthread.php?t=219014

I would not say it is not worth owning.

Regards,Steve