The problem with Justin.TV is the illegal content - the NBA Finals, etc. can't stay there forever. It can't survive on that - it'll get crushed. uStream's content is higher quality and legal.
Of course, maybe its legality just doesn't matter on the net.
In the case of youtube it seems most copyright owners tacitly accept the presence of their material, probably because they recognize the promotional value.
It's a weird situation now where people casually link to illegal material, like a Family Guy episode or a clip from Office Space, and nobody even blinks.
Of course, maybe its legality just doesn't matter on the net.