The NiteCore Extreme is a nice little light (only 92 mm) that will no doubt give you a lot of bang for the buck. You can get one for around $70,00. That's the price you'll pay for just the Malkoff M31 drop-in. There doesn't seem to be that much information about how well it will hold up under continuous recoil, as a rifle light.
There is one CPF-thread discussing it briefly:
http://www.candlepowerforums.com/vb/showthread.php?230150-Nitecore-Extreme-Weapon-Mount. Also Google: 'NiteCore Extreme weapon light ar15'.
It would help if you would tell us for what purposes you're planning to use it. If not wanting to spend too much money, for example for airsoft or fun shooting, I would have no problem trying the NiteCore Extreme out as a weapon light. But if you want something for defensive purposes, law enforcement or even military use, I second what angelofwar said in post 8... Get a combo with a proven track record of reliability (and available accessories).
Regardless of your specifications in your initial post, for defense/LE/military use, this is what I suggest:
Number 1: Buy once. Get it over with and buy a SureFire Scout (2x CR123) or Mini Scout (1x CR123).
Number 2: If the prices of the Scouts are too steep, choose either a SureFire P60 style light (e.g. the G2 with aluminum bezel or 6P) with Malkoff or an Elzetta ZFL-M60. A SureFire G2/6P with Malkoff-combination is around $140,00. The Elzetta with rotary switch costs around $160,00. These are fully upgradable.
---> 1 & 2 are your best bets, in my opinion. Proven reliability!
Number 3: If 1 and 2 are out of the question, and you specifically want a 1 CR123 light, use the Moddoolar TL34, or FiveMega 16340 Think2x proposed, with a Malkoff drop-in of your choice.
Number 4:...
In the long run, you'll probably save money buying one of the above (1-2-3).