I've got a Sky HD box but don't subscribe to the HD pack meaning I don't pay the extra £10 a month, still get the plus features, recording live pause etc and get all the FTV HD channels (BBC, ITV, Channel 4, FIVE etc HD channels) but not the HD channels that you pay for normally Sky1 HD, ITV2 HD.
Its defiantly worth it if the source program is in HD, but would never pay the extra as I don't really watch any other channels than the main five and channels that aren't in HD anyway. If you already have a sky dish and cabling and stuff and not bothered about the additional channels see about getting a second hand HD box.