Xenons?

Brighter than "normal" halogen lights to the point they usually install a lower wattage bulb when they are used i.e. the halogen bulb for the dip beam is usually a 55w job the Xenon is "only" 37w.

They are "supposed" to last for the lifetime of the car mine have so far lasted 2.5 years and 65k miles.

They are the sort of upgrade that once you have it's difficult not to want them on everycare afterwards a bit like the heated front screen for me.

Cons are they cost a bloody sight more to replace "if" they do go.

Oncoming drivers can get pissed off by them at times because of the extra brightness

There are currently only single element Xenons available to the best of my knowledge so you need a twinlight setup which generally does not seem to be a problem with new cars but obviously maybe if you want to retrofit them

All installations of Xenons require by law that the lights are self leveling and have some sort of lens washer installed obviously this increases the cost and the probability of things breaking down.

Having said all that they are the first option I would choose if not fitted as standard especially if you do lots of night time motoring and like me you have reached a certain age shall we say. My curent TDCi Ghia I specced them as an extra with no regrets my next car ST3 comes with them as standard and for which I am entirely happy.