Once you've committed to buying a film, you pay for it using Microsoft's Point system, with films costing from 250 points for a standard def version of an old movie through to 540 points for an HD version of a new title. This works out to roughly £2.10 for the cheapest films, up to £4.50 for the most expensive HD ones.
For this, you get to 'own' a film on your console for 14 days before it is deleted, but once you've started to watch it you then have to complete your viewing within 24 hours. Or you can, if you wish, watch the film multiple times within that 24-hour period.
This cinema-like approach is obviously a very different scenario to what you get with Blu-ray, where you pay much more up front, but then get to keep your film for ever, to watch as often as you like. Which approach you prefer is, of course, entirely up to you.
Having just mentioned differences with Blu-ray, I guess it's also worth me stressing that you don't get any of the extra features - director's commentaries, documentaries etc - with a film downloaded via Xbox Live that you tend to get included on a Blu-ray disc. But again, we know there are plenty of people who won't be bothered by this at all.
There's one more rather important difference between the Xbox 360 movie experience and buying a Blu-ray, too: time. For while a Blu-ray is available to play within seconds - depending on your player!! - of putting it into the disc tray, downloading a film inevitably takes time.
To give you some idea of how much time, using a perfectly normal BT broadband connection, I downloaded the 5.0GB file of Wanted in HD in one hour and 55 minutes. Clearly, then, if you want to watch an Xbox Live film, you need to think ahead - maybe leaving a title or two downloading when you go to bed the night before you intend to watch something.
I should add here that the Xbox service does allow you to start watching a film once a part of it has been downloaded while the rest of the download continues in the background. But you're still looking at a pretty hefty delay between deciding to download a film and actually being able to start watching it.
Mind you, I guess you could argue that unless you've already bought a Blu-ray you want to watch, the time taken to pop to Blockbuster to rent the Blu-ray could well be longer than it takes to download a film to the point where you can start watching it via Xbox Live…
With file sizes for full feature-length films on the Xbox service ranging between 4.5 and 7GB, anyone thinking of using their Xbox as their main source of films also seriously needs to consider having at least a 60GB HDD in their console (this is standard in the £170 Xbox 360 Pro) or preferably the 120GB offered by the Xbox 360 Elite.