Well, after years of driving my Jaguars, which all feature HID lighting systems, I guess I've been spoiled.
I found the halogen headlights in the 500sl woefully inadequate. Now, I was faced with two options as far as updating the lighting. First, replace the stock halogen headlights with factory HID headlights, which would cost approx $600 - $700 used depending on the source, or do an aftermarket HID conversion kit.
I started searching and went to a website run by Jim Powell (who I know from the bmw forums), called DDMTuning.com. HID conversion kits are $25 + shipping there at the moment. I decided I would give it a try as I really had nothing to lose if I didn't like them. They are plug and play, no cutting of factory wires required, therefore if I was unsatisfied with the results, I could always remove them and stick the halogen bulbs back in. So I ordered a 9006 kit, in 4300k - the brightest HID color, and the one used by all the OEM car companies. I know this isn't the best way to approach this, and that a factory HID conversion would be a better choice, but at the moment, there are other priorities I have with the car and I didn't want to invest that much into the lighting upgrades. All in all this entire lighting upgrade including HID headlights, and replacement fog light bulbs came in at around $50.
This is a picture of what the kit looks like from their website.
It took me about 35 mins to install - 10 of which were spent fighting with the stupid bulb retaining clips.
I had to move the washer tank on the passenger side to get better access to the headlight, but other than that it was perfectly straightforward.
Now, for the fog lights, I was suprised to see that Mercedes had chosen regular H1 halogen fog light bulbs for this car, and not the yellow fog lights that were common on european cars of this era. My 1990 190e 2.6 I had 10 years ago actually had a yellow glass lens over the bulb socket to achieve the yellow fog light look. I briefly considered converting the fog lamps to HID on this car (I've done it in my Jaguars), but after looking at the housing and the bulb clip, I decided against it. Instead I decided to replicate the yellow fog light look of the era, and went with a set of Nokya H1 2500k yellow bulbs. They fit the bill perfectly, and give a good contrast to the HID lighting.
After installing all the lighting, and taking the car for the first drive, the difference is night and day. I feel like the halogens were like having the parking lights on relative to the lighting these put out. I have driven the car for a little while now, and since my headlights are properly aimed, I have not had anyone high beam me or otherwise protest to the the lights on the car.
Here are some pics of the car in the driveway with the lights on.