If they're mainstream Hollywood movies, or well-known TV shows then they're not legal (if they're what they say they are)
Are they safe? Who knows? Basic media types (like wmv files, or mpg files) are safe, but if first you have to load a special downloader program, or a special viewer program, then that's like handing the key to your front door to a total stranger who offers to stock your shelves - for free - with all the very latest video releases. Out of the goodness of their heart. Might be honest... might not. Up to you.
Bookmarks