As the two other guys stated, those Xentec HID kits are pretty cheaply made. They come with basically no instruction, and the kits I bought didn't come with a headlight harness plug that is unique to your car. Instead, it has 3 small leads that they expect you to just stick into your headlight harness and pray they don't fall out while you are driving. I did find out that you can get a harness plug at autozone/oreilly/advance to lock the three leads into place, and it actually works pretty well. I've had three sets of Xentec HID's, and it honestly just depends on which box they send you. My first set lasted me almost three years, second set two months and I've had my third set in for about a year now. They are extremely temperamental. Just a slight bump can knock the ballast the wrong way and bam, out go your lights. Also, the bulbs seem VERY fragile to me, even moreso than halogens.
NONETHELESS: I made the decision to buy these because they are $40 a pop. I figured I could have at least 5 sets before it added up to what a high end kit would have cost. When they work, the Xentecs are crazy bright. So if you're willing to have to reposition ballasts every month and a half (and potentially by another $40 kit in 9 months), I'd say go for it.
09 Mustang 4.0. Too much to write down. Looks and sounds like a GT500 for half the cost and a quarter of the horsepower!! Currently working on a junkyard turbo build and an 8.8 rear end swap.