iirc, if you have sky+, you pau £10 per month for that, if you get HD, it's still £10 per month and includes the "plus" functionality for recording, etc... so you're no worse off, except you have to find a box and install it, or, pay sky for a box and installation.
I bought my box off eBay, simple job to unplug SKY+ and plug HD, nothing to it really.
HD rocks, the picture is stunning, you may end up watching programmes you didn't really care about in the past, just for the quality of the picture
Note: Sky one is on HD, but not all of their programmes are actually in HD.
The amount (i.e quantity) of HD content is quite poor to be honest, one of the HD movie channels doesn't even broadcast 24x7...
All in all, I don't regret getting it