I've got an HDTV setup here and there are a couple of demo channels live. It is absolutely beautiful. I only have a 30" screen (it's a monitor, not a telly), and it looks perfect from less than 2' away. The picture is physically four times the size of DVD resolution (twice as wide, twice as high), but unfortunately the bitrate is about 4 times as high, too, meaning if you want to record a film in HDTV, you're looking at 15-20 GB HD space (or 5 DVDs).
The World Cup will definitely be available in HDTV, as Premiere are already bigging it up, and I'll definitely be ordering it soon
I'd recommend waiting a year or so before getting HDTV: so far only the odd bit of sport and some US series are available in HDTV (if you can even receive it) and the technology to save your recordings isn't available yet (HD DVD, Bluray). I already have the setup I need here and HDTV is available from Premiere, but I haven't ordered it yet because I have no idea what to do with the recordings.