nau - with short lenses 1.4 indeed doesn't give You too much, but with anything above 400mm gives You a huge increase - 400mm lens becomes 560mm, it also alows You to use AF with some lenses, i.e. f2.8 lens will work with pretty much all TC on the market (TC14, 17 and 20), f4 lens will work only with TC14 (not exactly true, but at least officially that's haw it goes); also, the higher magnification the more IQ You lose - as TC's are working as a magnifying glass

This brand strangely enough has a 1.4 and 1.5. I have both.

dont really see point in 1.4 since it gives u minor increase (very minor)
would love to have 1.7 but its not available for this brand : )

any one with 2x ?

I have the 1.4x version. Works nicely. Mates with just about any lens. But as with all TCs you need good sharp glass. I use it on DX and Film(fx). I think its max aperture is F1.8 which is not too bad. Really nice. Highly recommended TC.

I use it on DX but not FX yet.

any one used Kenko Teleplus Pro 300?
on FX ?

what do you think ?