I've heard they are ok but the download takes forever and you only get DD/DTS sound, no HD audio. I think the picture is 720p too. I personally would get a subscription with Lovefilm etc and save your money.
On a price comparison, Lovefilm wins outright (£5.87 gets you 3 films per month). However, it may well take a long time to download an HD film, but it is still quicker than Royal Mail (just). As indicated though, the Blu-Ray will without doubt be much better quality.