jeneric21-i agree with what everyone has said, but there are several more things to consider for I have gone thru the same thought process and I am still without xenon.
non-xenon cars do not have the high pressure headlight washer and it is not feasible to have it installed. xenon light is much more concentrated and more easily refracted thus the importance of having the headlight lens clean and free of debris.
non-xenon cars to not have the auto-leveling/auto-adjust feature built in and it is not feasible to have this installed either.
lastly, i am not certain if the wiring harness are the same. if you can just unplug the halogen unit and then plug in a xenon unit...
as for what Mike did, i believe that is a conversion and he did not change out the headlight housing. i don't know about the euro-spec cars, but the US cars use different headlight housing units in halogen vs. xenon equipped cars. therefore i do not know how conducive a halogen headlight housing will be with a xenon conversion.
no doubt xenon looks cool, but due to these questions and doubts about "true" lighting performance gain, i have yet to buy anything...i'm just not sure if it is worth the trouble and money