If you've already got an dish up then just buy the box and plug it in, theres nothing to pay or do other than buy the box. Its not quite the same mix of channels as freeview – you get all the BBC channels (3 and 4 News 24) and the Channel 4 ones (E4, Film 4, More 4) plus the usual jumble of shopping channels, rolling news channels (Al Jazera is really good, surprisingly) and channels of corny US repeats. You also get all the BBC digital radio channels and all the regional variations of BBC1.
What you don't get over freeview is any of the extra Channel 5 channels (Fiver) or any of the BBC owned commercial channels – all the ones that used to be called UKTV and are now called 'Dave', 'Yesterday' etc.
If you do decide to go down the route of buying a recorder, which I recommend actually they change your TV habits for the better, then the other thing you need to be aware of is that the recorders require two leads to run from the dish to the box, so if you have an existing dish you might need to have a second cable run from it and I'm not sure all dishes are designed to allow that. Unlike Sky Plus its also free to use (with a Sky+ box even once you've bought it you need to pay a subscription to use the record function, regardless of what channels you do or don't pay for).
