HID
I have HID-ified the main beam and the low beam, and have also HID-ified the 2 BMW fog/driving lights that came as a £200 optional extra on the 12 GSA.
One good part of that is the lights are already fitted and you only need to fit the HID kit. Also good is that they can be used on low beam as the fitted foglights cut off the top half of the beam. That means you are driving around with 3 HID lights on at all times ... around 10,000 lumens of output ... as opposed to about 1300-1500 lumens from the standard single halogen bulb. Its a massive difference.
The main downside is as they are foglights rather than driving lights, you only have one "high beam" light out of the four when you switch to high beam.
Having said that, driving at night on the HID high beam (and with 3 HID low beam lights on as well) not many people are going to be screaming for more light in front of them ... there is a lot that this setup puts out.
So far the setup seems durable. The HIDs were put in after the first few hundred kms and I have since done about 10,000km (6k miles) around Europe on the bike in the last 3 months on both sealed and unsealed roads, and no problems so far with any of the ballasts or HID bulbs. .... so far ...