Got tinted windows on two cars, my 1986 635csi and my father 99 528ise. The windows are made darker by adding a security film (Like sticky back plastic). There are various different shades ranging from uncloured to midnight black. It is illegal to have the windscreen coloured other than a top stripe and the front side windows must be no darker than light smoke (about grade 3 of 7 shades). These rules mentioned above are now laws and if you have darker than legal requirements the police may ask/tell you to have the film removed. My 6 is dark smoke and was fitted by the previous owner in 1988, whilst the 5 is medium all round, but you can get away with a medium smoke in the front windows if you have a very light interior as it will look lighter which is what we have.
There are two types of films that can be purchased. The cheap stuff that you buy and fit yourself which is pretty horrible and is difficult to see out of and American Window tints which are professionally fitted. The american glass allows you to see out of, but makes it difficult for people to see in. Yes the process is reversable, but its not easy, as A: Soap and solvents will get all over your interior, B: It's as exspensive as fitting the tints and C: The removal process may well damage your Heated rear window element which is why I have not had the film removed from my six untill such time as I am told to do so.
Benefits include less heat gain in the summer. Less UV radiation for not only yourself, but your interior too. Effectivly all windows are laminated and so in the event of an accident you will not be injured by shards of glass. Shatter resistant, and prying eyes find it more difficult to see what you have inside. Cost will range on the vehicle, but range from £250 - £500. Pentagon are the most famous fitter (And most exspensive). No a good company based in Sutton Surrey and can give you more details if you want. Alternativley look in your local Thompson/Yellow pages.
Hope this Helps
James