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.