I've had HD since launch and the football's great, movies are very good, sky one OK, I really like the BBC HD stuff (probably watch that more than anything).
Picture quality is important to me and I get more enjoyment from a film or match if it's in HD so for me it's worth £10 a month (the HD box had a big memory too so you're unlikely to run out of space if you record mostly SD stuff). For ther majority of people though I'd say don't bother yet.
It's good that Channel 4 have launched a channel but it looks to me that most of it isn't proper HD.
Think about what you watch most of, for me it's football, movies and some natural history, if you're the same I'd say £2.50 a week is worth it. Otherwise, don't bother yet.