The biggest problem with people dropping 'drop-in'/'plug-n-play' HIDs in cars that did not come with them is not so much the HID itself, but the optics they're putting them into.
Don't get me wrong - there are TONS of really [censored] HID kits out there, which either put out poor light, the wrong 'color' light, etc.. but the big issue is that HIDs really only work right when they can be aimed well.
What this ultimately leads to in a ton of cases, is people putting HID kits into cars with reflector type headlamps.. which just end up throwing the light everywhere, and therefore blinding oncoming traffic. Even put in cars with projector type headlamps, a poor quality HID kit, or even one where the HID 'bulbs' are not properly centered, will put light where you don't want it. It's harder to find a set that works than you might believe.
{{ Flame suit on }}
Having said all that, I did put an aftermarket HID kit into my 2010 Fusion about 2 years ago. I did this after thinking long and hard, weighing the positives and negatives, and committing to only keep them in if I could honestly say to myself that the light output was:
1 - At least as bright as the existing halogen bulbs, with a prudent color temp (not too blue)
2 - properly directed / cutoff at the appropriate level, so as not to cause glare or blind oncoming traffic
3 - reliable
Keep in mind, the 2010 Fusion has a decent projector headlamp from the factory, so I felt it was worth a try. I also went with the 35w kit and not a 55w kit.
Again, this was two years ago.. Before I put the kit in, I lined the car up facing my garage door and marked off where the light hit, and then proceeded to install the kit and compare the output to where the light hit from the halogens. The cutoff was very close, although there were a few 'stragglers'.. I felt it was close enough to test more. I had a friend drive in another car, and we tested various scenarios.. me driving behind him various distances, passing going opposite directions, etc.. He said there was no glare so I felt it was a successful install.
That was 2 years ago. I have not had any oncoming traffic flash brights at me, so it would seem the test was a good indicator. At this point, however, I'm starting to see intermittent issues with one or the other lights not always coming on when I start the car, so I suspect the ballasts are either failing, or the 'bulbs' are dying.. I will be replacing them with standard Osram halogen bulbs, and I will not be going back to HIDs. My experiment was a decent success, not good enough. I just don't feel that there really is all that much of an improvement going to HIDs.. at least not in this car.