If you don't have washers, auto levellers and projectors (most headlight housings are reflector, not projector) they don't meet ADR and you can be defected for them.
They don't give you a proper cut off with reflectors so you might have really bright lights (not a good thing in bad weather either) but the light will be going all over the place, any dirt on the outside of the headlight will effect it more so than a regular light globe also.
TL;DR version: HID's are *poo* unless they came with your car standard.