Well, for a start they're a great deal smaller than yer average S_y box. Maybe a quarter of the size. It also consumes less electricity - so if you leave it on 24*7 (which you almost certainly do with the aforementioned) then instead of pulling 35Watts, which will cost you £40 or more over the course of a year, you use much less power and save money.
You can also stream the programme it's receiving onto a PC from the DM's web interface. This can be done over a wifi connection, too - so much more convenient.
Oh yes, as well as receiving telly - if you have a collection of movies in AVI/MPEG format, it'll play them. You can even have your PC networked to a DM and play the films off a shared drive (NFS or SMB) - same goes for MP3s.
You don't have to be a tinker-er to use one of these. It's quite possible to just set it up once and leave it alone, next to your TV and use it simply as another domestic appliance. While a lot of people have a hobby of fiddlin' with their DMs: upgrading, re-imaging, customising them; you don't have to - you can treat them just like you would a fridge or a hoover: there to do a job and nothing else.
One thing on the downside though. A DM only comes with a single satellite input (you might be able to get a second, but that would mean fiddlin' with it internally) so you may be used to watching one programme while recording another - this isn't always possible with a bog standard DM.
