Size/Weight:
The Quark X 123*2 is a great size, nice and small but large enough to fit in your hand nicely and be easy to use. I found it was even light weight enough to hold in my mouth if I needed two hands for a task.

Grip/Handling:
The knurling is like most the other Quark lights of the past, fairly smooth but still provides some grip. I noticed myself getting most the grip from the pocket clip, which by the way is removable and even reversible. You can mount it on the tail or head end. This is a nice feature that makes the light a great fit whether you like carrying your light head up or tail up in your pocket.
One thing I did notice was that when picking up the light in the dark, it was alittle hard to tell just by feel which way it was orientated in my hand, since the head and the tail end are the same diameter and feel very similar. That is the case with alot of lights that dont have a larger diameter turbo head though.

Build Quality:
Like all the 4Sevens products, I see no issues with build quality. The machining is very well done, no sharp edges, the black anodizing is smooth and even. The LEDs were nicely centered in the reflectors and I noticed no dust under the lens. The pocket clip is very stiff and feels very sturdy.

Output & Runtime:
    With the use of the new Cree XLamp XM-L LED the new X Series has an improved output without sacrificing runtime. Both my test units had a warm tinted hot spot that fades smoothly into a cool tinted flood. The differences in tint are only noticeable when white wall hunting though. In normal use I liked the warmer tinted center, it added some color retention.
One of my favorite things about the X Series is the Moonlight output. There are way too many lights out there that have their lowest output setting way too bright for fully night adapted eyes. All the other output levels are also very nicely spaced and usable.

User Interface:
The user interface for the Normal and Tactical versions of the Quark X series are totally different, and at first I had a hard time deciding which I liked the best. I ended up liking the Normal version better.
I will try to explain the differences the best I can, but basically the Normal version is best for those that like easy access to all the lights output levels. The Tactical version is best for those that want a light to operate much simpler, have easy access to just two output levels and dont need the access to all the output levels easily. But would still like to be able to customize which two light levels the light is using by default.
I liked the fact that the Normal version would always turn on in Moonlight mode. I found this a better solution than the light remembering the last output level I used because I dont always remember what level I last used. Plus this way the light does not blind you if you turn it on with might adapted eyes. If you need a higher output it is quick and easy to lightly press the switch a couple times jumping up in output levels. Or just twist the head tight to jump straight to the max output level.
The Tactical version is much simpler in use, but a bit more tricky to setup which output levels you want it to use.
Basically you have two output levels that you will have easy access to. One with the head loosened and one with the head tightened.
To set the output level you have to tighten and loosen the head at least 4 times quickly, stopping with the head in the state you want to program, tight or loose. After three seconds the light will blink indicating it is ready to be programmed. Now turn the light on and off with the tail switch until you have the desired output level selected. The Tactical version has a forward clickie, so you will have to fully press with switch turning the light on and off with a full click. Each time you turn the light on and off it will cycle to the next output level.
Moonlight > Low > Medium > High > Max > SOS > Strobe > Beacon
After a few seconds the light will blink indicating that the output level has now been set and memorized.
Normal Version:
Reverse Clickie Switch (no momentary on)
Able to Tail Stand
When Bezel/Head is Loosened light will always turn on in the Moonlight output level
Lightly press the switch (no need to make it click) to cycle through the output levels
Moonlight > Low > Medium > High > SOS > Beacon
When Bezel/Head is Tightened light will always turn on in the Max output level
Lightly press the switch to cycle back and forth from Max and Strobe
Tactical Version:
Forward Clickie Switch (has momentary on)
Not able to Tail Stand
When Bezel/Head is Loosened light will turn on in the output level set by the user
When Bezel/Head is Tightened light will turn on in the output level set by the user
Pressing the switch lightly will operate the light in momentary mode, fully clicking it will turn the light on and off.

Pros:
Good output and beam pattern
Sturdy Removable/Reversible Pocket Clip
Able to tail stand
Evenly spaced output levels
Very low Moonlight level
Good build quality

Cons:
Switch is sensitive, may accidentally change levels if using the light with thumb rested on switch
Not able to use an 18650 cell (though this would have meant that using 2×123 cell would have rattled)

Conclusion:
The Quark X series of lights is another great addition to the always improving line of 4Sevens lights. They do a great job of listening to what the customer wants and offer different versions to meet a persons specific needs.