rear projection

Hi all, I am investigating the possibility of using images projected on to a screen as the backdrop for one of our upcoming productions. We have not used this method before, so I am trying to get more information abotu it. Does anyone have any experience using rear projection? Opinions, etc welcome! I have heard it is "expensive" but I would like more details! What does it cost to hire a projector (with a decent lens, as backstage space is limited).