I installed some Philips MotoVision bulbs. They are H4 65/55W bulbs, same as the standard ones in the Strom so it's a straight swap and you're still legal. They are claimed to give 40% more
white light, have a 10-20m longer beam and make you more visible. So far I can say that they are brighter and I'd say the claimed 40% figure is probably accurate. As for the longer beam, I seldom use brights and I think this would be the only time you really notice it, I haven't.
The real beauty though is being more visible. Although the light is whiter the bulb has an amber tint on the glass and this gives your headlights an amber glow/look. The first day I used them I thought it was my imagination, but from the second day onwards I was sure that the bulbs make a difference. People were definitely noticing me more and more car drivers than ever pulled over to the middle lane as I approached. Another bonus is that the amber tint should reduce the amount of blue light. This is a good thing as blue light refracts most and contributes to people thinking your lights are too bright. Because blue light refracts the most it is also the worst light to have when there is mist/rain. So not only are you more visible, your slightly amber lights gives better visibility in misty/rainy conditions.
Here's a link to an album with some pictures, they have captions.
IMHO this is one of the best upgrades and it's fairly cheap. I got mine off Ebay, £22.18 delivered for 2 bulbs.
Here's a link to the auction, the seller was friendly and quick to respond when I asked him to confirm that the auction is only for 1 bulb (it is.)