Depends on the digital copy, I think. The digital copy that came with Constantine can work with iTunes, which allows you to authorize the movie to your account, and then it's just like any other purchased/authorized iTunes content, where you can copy the file around to as many computers as you want but it will only play on up to 5 computers that are authorized to work with your iTunes account. Works well for me, as 5 computers easily covers my HTPC (an iMac), my two laptop, my wife's laptop, and even my work computer. It also plays on my iPhone, which does not count towards the overall computer count.The digital copy is actually for playing on a portable device like an iPod as well. The quality is not nearly as good as a BluRay or even SD DVD but looks fine on the small screen devices that allot of us use. My understanding also is that the digital copy will only work if you have the original disc in the drive when putting it on the device.
The quality (640x480) is not quite as high as it would be if I ripped a DVD instead (720x480), but this difference doesn't show up on the small screens. I can tell the difference when I watch on my AppleTV, however (another device that doesn't count towards the 5-system limit), but it's still quite watchable.