I'm cautiously optimistic but I do have concerns with how much it could degrade the bare lens when shooting it at the 1x position. The elements are flipping out of the way like with a built-in TC lens. They are still in the image plane, just shifted forwards or backwards. I'm not a fan of 2xTCs anyways so this device would only have value to act as a 1x to 1.4x converter that could be used on existing lenses without having to upgrade lenses to a new "built-in TC" version at extra expense.
But I think I'd still prefer a simple built-in 1.4TC as we have in certain lenses already. Lens designed specifically for that 1.4TC to be inserted and removed into and out of the optical path. Not a do it all, bolt it to any lens, and somehow expect those additional elements aren't going to degrade the native lens' IQ.