It's incredibly difficult to send very high-quality pictures over the Internet -- broadband connections may have become much faster over the last few years in the UK, but they're still not that fast.
I've seen a fair few 'TV over the Internet' services over the past few months, and while the picture quality on some of them is more than watchable, none of them have reached what I consider to be true TV quality.
The key element will be the codecs Joost uses. If the founders have worked out a way to compress video further without losing picture and sound quality, they could be on to a winner.
Once myat home, I'll try out the service and see how good it is. Fingers crossed.