Just fitted mine. I used a Techone kit recommended by other 156 owners.
The bulbs didn't really fit properly, as they're longer than standard H7s and fouled the metal cap that's fitted within the headlight to reduce glare.
I could push them in hard and fasten the bulb clips, but the beam pattern was wrong and I worried about the effect on the bulb of operating it when it was pressed hard against the metal end cap....
So, the only way I managed to fit them properly, was by removing the bumper and then each headlight.
I then removed the metal cap from within each headlight and used various implements from my garage to tap out the end of the metal cap. (The metal cap has a flat end - by holding it over a ring spanner and tapping it out, you could turn the metal cap into a "dome" shape, giving you 2 or 3 more mm of clearance.
(The above took about ten hours from start to finish - and I'm still wearing the plasters on my hand caused by my angry 156 - its vicious!)
The other hassle with fitting discharge bulbs is that you have to drill a hole to feed the new wires into the headlight from the ballast unit. This can look messy. The other benefit from taking the headlights out, is that you can drill the hole for the grommit and wires in the bottom of the headlight unit, so you can fit the ballast without it looking untidy after you've put it all back together.
Bottom line is that I've spent my money and bought the kit, so I had to make it work.
Looking back, I should have saved my money and kept to my Osram Silverstar H7 bulbs that I had previously fitted.
The 156 headlight reflector is particularly poor and is never really suitable for HID bulbs. The beam pattern's not the best and there's always loads of "scatter", which is exactly what you don't want with an HID bulb.
By "jiggling around" my Osram Silverstar bulbs, I managed to get a decent beam pattern that I've not been able to repeat, either with the standard H7 bulbs, my previous Phillips Vision Plus H7 bulbs or the HID kit.
So, knowing what I know now, even though I now have HID units fitted, I should have saved my money and kept my Osram Silverstar bulbs....
That would be my recommendation to any 156 owners, unless they want to replace their complete headlights with the proper projector-lens type that became a factory fitted option in 2002.