Recently bought the ATX 30-70x95 and 1.7x.
First tests with 1.7x in the evening (it was already slightly dark and cloudy) were encouraging.
Of course at 120x it is a bit darker than with with 60x, 85x or 100x (with extender) but it is still ok, even in the evening. Sharpness is excellent, at least when there is no haze or hot air. I watched a Black Redstart from about 25m distance and the detail and sharpness at 120x were very good. The same with a Carrion Crow at about 80m and a Roe Deer at about 400m.
It's probably not useful in Spain at noon on a hot day watching a distant Bonelli's Eagle
Turning the zoom into a 50-120x is probably not great for scanning large areas, e.g. scanning fields in eastern Austria for sitting Imperial Eagles. But once the eagle is found, the 120x will provide a whole new experience.
Adding or removing the extender is pretty smooth. That said, it would be cool if it were built-in into the ocular with a simple switch to switch it on or of (as in the Canon EF 4/200-400L IS 1.4x lens).
A sturdy tripod is a must. I use a Gitzo carbon tripod and a MVH500AH head. The head is new, not much experience yet but first impression is good.
If you already have an ATX scope, then getting the extender is definitely a good idea.
I've long thought about getting the ATX 30-70x95 and the 1.7x finally made me buy it.
I often look at distant birds, particularly raptors. So the 50-120x makes sense. If you mostly watch birds that are pretty close, e.g. monitoring woodland birds, than a 20-60x makes more than. But even then, once you've found that perched Black Woodpecker, you may want to zoom in to 120x
I'll post more once I have tested it more.