Now that it's released I'm not entirely sure how I feel about it.
On the one hand it's a perfectly serviceable model - well, not quite, I still have a few problems with it from an aesthetic standpoint, but I can see potential and I've already identified just how I'd convert it to fit my liking (which would be a fairly easy task in itself - some shifting and modifying of parts here, a little bit of reposing there and I'd be in business). The rules aren't too bad either from what I can see, it might not necessarily be a titan-killer, but it looks like a pretty-decent Knight-buster, able to stand toe-to-toe with the average Imperial Knight and stand a good chance of coming out on top, with less raw power in melee being offset by a better protective shield and more ranged firepower. I'll be interested to see what innovative tactics and combinations members on here come up with as they explore its capabilities further.
On the other hand, I'm not entirely sure I'm sold on its background. The first glimpses of it I saw suggested that it was primarily intended for fighting in extreme environments too harsh for conventional units to effectively operate in, and I could definitely get behind this idea, indeed I suggested something along those lines as a possible explanation some time ago (in essence under this idea it would sort of be a Tau equivalent of the Object 279 experimental tank prototype the Soviets developed in the 1950s). The description on the Forgeworld website suggests they have gone with the somewhat simpler concept of 'direct counter for bigger nastier enemies too much for all the units and weapons currently in use'. I personally find this a little bit boring, and as an Air Caste admiral at heart I found the online description a bit smarting (Our aerospace craft outclassed? Bah! Give my strike craft crews a superheavy target and they'll turn it into a smoking crater inside of 30 rai'kor, air defences or no!). I'm also a bit confused by the description of the pulse ordinance multi-driver, It was always my understanding that the 'pulse' part of pulse weapons came from the plasma pulse bursts they fired - surely using induction fields to propel solid munitions would make these some sort of mass-driver rather than a pulse weapon?
The more I think about it the less sure I am I'll get one. Don't get me wrong, I can definitely appreciate how others can like it, and I'm very happy for those who are pleased with it (and it's always good to see some xenos love from Forgeworld), but more and more I'm thinking it just isn't right for me. As much as I always daydreamed about Tau titan equivalents when I was younger, at the end of the day I've come to really like how the Tau solution to superheavy units in the past was always an asymmetrical one rather than a symmetrical one - defeating the carrier group with a barrage of anti-ship cruise missiles launched from bombers and submarines rather than dukeing it out with another carrier group, to use a Cold War analogy. The more I think about it, I just don't think it fits with the character of my army - my hunter cadre's commander is a very 'grass-roots' character, the sort of military leader who prioritises good communications gear and supplies over prototype superweapons, values small-scale support weapons and equipment and generally thinks there's always an important place for the common infantryman no matter what, and who tends to defeat giant scary things asymmetrically with smart tactics and good leadership rather than sheer brute force. The more I think about it, the more he probably wouldn't see much use for something like the KX-139, and as a storyteller first and a hobbyist second that in-universe thought process is an important one for me. The more I think about it, the more it just isn't my style.
Of course, all that said I can definitely see how it would fit well with other member's armies, and the great thing about the current situation is that both approaches are (on paper at least) viable. Really as long as my trusty bread-and-butter staple units aren't overshadowed too much, I think everything should be just fine.