Actually, there is. It's one of my threads in the W220 forum, featuring the E320 CDI in one of the posts.
Here's the page that shows, as an example, halogen high-beams vs. HID high-beams. Low-beam performance is similar. Look for the post comparing the S430 vs. the S500. The S430 here has "single xenon" low-beams, and traditional high-beams. The S500 has the exact same headlight assembly, but upgraded by me to bi-xenon (low- and high-beam HID).
And here is what my E320 CDI's new headlights look like. I hadn't taken "before" pictures on this car, but the difference was *stark*, I assure you.
We also have two Honda Civics. I have converted one of them from halogen to HID. Compared to the other Civic of similar vintage, the HID-equipped Civic is so far and away brighter that they're not even in the same conversation anymore. Really.
Here's what the Civic looks like now.
As for reliability, if you get junk, yep, they're unreliable...just like with anything else. I admit that I made that mistake on the Civic in one area (the bulb-to-ballast adaptor), since replaced with a quality adaptor that is rock-solid. Amazing what just $5 more will do for quality. :-) And that's really the key; if you stick with quality stuff, e. g. Hella Gen 3 ballasts, Osram/Philips/Jahn bulbs, and either Hella or Morimoto projectors, I've found the HID's to be very reliable, indeed. I love 'em, and there's no way I'd ever go back now.
But aren't the bulbs expensive?
Not much more than a pair of those "xenon" halogen bulbs, really. Expect to pay, real-world, between $60 and $100 for a pair of good, non-Chinesium bulbs. I go into this extensively in the aforementioned thread. Remember, though, that HID bulbs will last 17 years. How do I know that? Because the project 2000 S500 was 17 years and 135,000 miles old when I bought it, and it was pretty clear due to the "blue shift" that those were the original
bulbs. Yeah, baby. That's why I stick with Osram, Philips, and Jahn. You'll likely get at least 15 years out of your bulbs, perhaps more.