It's not bad. UK Gold is quite good if you like that sort of thing and E4 is good if you like Friends and Big Brother. There's a few other channels, but for me, those are the best. But before you look into getting it, be aware of a couple of things. Firstly, the channels they carry don't all have the same transmission hours as the channels do on another platform, like Sky Dig. Each channel has to share it's spot on your digital box with another channel so that they have enough room to fit them all in! So make sure that the channels you want are available at the times you want them.
Secondly, it's not an add-on for Freeview. It's a different service altogether. It's Digital Terrestrial Television like Freeview, but that's about it. To view the extra channels, obviously you have to pay for them. Once you subscribe, you'll receive a viewing card which you need to put in your DTT box. Unfortunately, as far as I know, every 'Freeview' box made so far, doesn't have a viewing card slot! At present, it's only really the boxes that were used with On/ITV Digital that can take the cards. Many IDTV's also have problems. If you go to the website - www.topuptv.co.uk, it'll tell you everything else you need to know including what boxes/IDTV's are compatible.