UPDATE: I am aware that the PTR has been updated today (Sep 23), and some of the “clunky” and “underwhelming” aspects have been significantly improved. I am very happy about that! 
I played Xal’atath vs bots on PTR multiple evenings in a row.
First of all, I’d like to thank Blizzard and the HotS dev team specifically for reviving the game in the traditional sense. Xal’atath will be an amazing addition to the hero roaster. That being said, I’ll focus this feedback on exclusively negative aspects of my Xal’atath experience, as I believe they are the ones that matter in practice.
Gameplay.
General.
In her current PTR state, Xal’atath feels considerably unresponsive (aka “clunky”) in her gameplay. This ultimately comes down to the slow nature of her abilities and bad feedback for the player. I’ll elaborate on this more while talking about each individual ability.
Abilities.
Void Volley (D) — this is easily the most frustrating of all Xal’atath abilities. The frustration comes from a number of factors:
- Extremely slow startup. The trait has a brief casting time, and, once cast, has a considerably slow projectile startup animation.
- Lack of visual indicator. The point on the ground where the trait will be firing to is not marked in any way. This makes it difficult to understand in the heat of the moment whether the trait has been cast or cancelled.
- Mythic Quest being tied up to the trait. The aforementioned two factors makes the trait feel unreliable and make Xal’atath to expose herself to more danger than necessary. This, in turn, leads to an unnecessarily increased difficulty in completing even the first step of her quest.
Shadow Mark (Q) — this ability is supposed to have an entire build around it, based on the talents. However, I am forced to discard this build entirely due to how extremely slow the projectile is. I played against bots, and I understand that bots are programmed to dodge projectiles. However, it gives a good idea on how difficult it is to hit. And, on top of that, there’s a center hit requirement to mark someone. A center hit requirement… on a projectile that’s impossible to hit in its entirety.
Dark Heart Ritual (W) — this is, in my opinion, her most useful ability so far, especially with the lvl 1 talent for it (that increases its size tremendously). The ability is great for PvE, has great talents, and can defend the channeling objectives like Altars or Tributes for considerable time.
Void Step (E) — this is an amazing idea for a unique mobility option. However, it has two major and, may I say, artificially created downsides:
- It requires a set target that (apparently) cannot be set or changed during the blinks.
- It disables all abilities, leaving only a basic attack option until the ability is complete or cancelled. This is particularly crippling to the whole concept of ranged assassin. I recommend considering enabling all basic abilities, trait, and even the Void Eruption ultimate cast (pausing the blinks for the channel duration). In the worst case scenario, please let us at least cast her trait during the blinks! An assassin is supposed to be all about “in and out”.
Void Eruption (R1) — great concept for her ultimate ability, but it slightly lacks in its current execution. While it’s great to be able to blind and damage while channeling the ability, its usage is currently very situational and is likely to result with everyone just running out of range. Possible improvements would be increasing the projectile speed or giving an ability to detonate it at will.
I understand that the ability is literally a giant blue ball, but that should be no excuse for blue-balling a player. 
Lvl 20 upgrade is awesome though!
Void Convergence (R2) — another great concept for her ultimate ability, and it’s good as it is, except for one detail: Void Minions attack marked heroes, but not marked monsters, which makes this ability a lot more awkward in boss fights than it needs to be.
Bugs.
Ultimates disappear.
Both her ultimate abilities can simply fail to appear on the map despite triggering a full cooldown. Apparently it’s related to the terrain (they get blocked by it), but I have the same bug happen to me multiple times out in the open as well (though it’s a lot more rare).
Attack speed bug.
Her “Relentless Whispers” lvl-16 talent gives her tremendously increased attack speed for the duration of her E. However, if her E is interrupted by any means, she keeps the increased attack speed until she uses E again.
Enemy Xal’atath’s Q.
Xal’atath’s Q moves not only her W, but the enemy Xal’atath’s W as well. I did not encounter this bug myself (bots don’t seem to play Xal’atath), but I saw it in content creator’s videos.
Silence area.
I don’t know whether this is intentional or a bug, but her triangular silence area (from her lvl-7 “Silence of the Lamb” talent) is not aligned with the triangle of her E.
Heartseeker skin’s visual bug.
Only when wearing the “Twilight Xal’atath” skin — on the MVP screen, the default void orbs on Xal’atath’s shoulders are displayed.
Skins.
I personally really like her “Twilight Xal’atath” skin. However, it would be a very nice touch if the skin also changed the hue of all her abilities from blue to the same red-orange hue of her inner cloak.
Please consider.