I bought the Tamron 1.4x Teleconverter because I'd rather loose one f-stop, than two (which is what would have happened had I bought the 2x one).  I use it mainly with a Canon 70-300 IS USM lens and on a sunny day it works great and auto-focuses with no problem at all.

However, I've found that on a heavy overcast day, or a rainy day, or in the early morning, or late evening, (anytime you have "low light" conditions) the Canon lens finds it harder to auto-focus with the 1.4x attached.  But, by using the manual focus ring on the Canon lens, I can get a fairly sharp image.  Usually, I'll just snap several pics as I slowly turn the manual focus ring on the lens.  One of them will usually be in good focus.

In short, I'm enjoying the Tamron 1.4x Teleconverter, but more so on a sunny day.
I bought the Tamron 1. 4x Teleconverter because I'd rather loose one f-stop, than two (which is what would have happened had I bought the 2x one). I use it mainly with a Canon 70-300 IS USM lens and on a sunny day it works great and auto-focuses with no problem at all. However, I've found that on a heavy overcast day, or a rainy day, or in the early morning, or late evening, (anytime you have "low light" conditions) the Canon lens finds it harder to auto-focus with the 1. 4x attached. But, by using the manual focus ring on the Canon lens, I can get a fairly sharp image. Usually, I'll just snap several pics as I slowly turn the manual focus ring on the lens. One of them will usually be in good focus. In short, I'm enjoying the Tamron 1. 4x Teleconverter, but more so on a sunny day.