My personal experience with X was not that lucky. I had my first one exchanged to brand new due to several problems like scratched casing, popping sound from the casing, bad battery and horrible display uniformity (screen had noticeable different hue between upper and lower portion) etc. My second one had pretty decent display (not 100% good but acceptable, except considering the price of this unit but that's what I guess is just extra you have to pay for Apple brand) but this one is very slightly bent, it had couple dents in the steel rim where space gray colour had zipped off, sim tray is not completely flush (it actually feels like it is slightly too deep in one side or seam between tray and chassis is little rough), still rather weak battery and something else I cannot remember right now.
Anyway, I decided to keep this unit because I'm not that lucky usually and since this one had acceptable screen and I'm keeping it in the leather case those imperfections are hidden, plus my dealer compensated these faults by giving small refund.
However, another family member bought also X at the same time as I from the same store and that one was problem free in every way. It is silver colour and from the different factory than my both. That is 100% good in every way. It has excellent screen and I'm very picky when it comes to display devices and I can honestly say it is excellent, much better than in my current one. Also no imperfections in casing at all and battery is incredibly good and it has not weakened at all in 3-4 months even it has been abused since person using this often forgets to charge it and battery has gone very low for many times but no damage from that and this phone can easily last 1-2 days longer between charges than mine, even I have babied my battery from the start, it keeps weakening.
So, I guess you need to ask yourself do you feel lucky? Are those two things the only problems in those units and they were otherwise perfect? If answer is yes, I'd say you were lucky twice already. I say consider this a little like playing Russian roulette.

From what factory yours are? You see that from the first two letters of the serial number.