What size is your TV screen?
To be honest the SD broadcast on many channels is pretty good to excellent, so there's little benefit to going HD. Plus the specific HD channels aren't full HD anyway (neither Sky nor UPC because of) so if you have a monsterous sized TV it won't be that great. And HD is only on the specific HD channels, all the others are SD and so no difference with the standard digital box.
If you're not a massive TV watcher I wouldn't say it was worth it and if you're only thinking of getting it for movies as some do, then Bluray is currently the only full HD device.