Personally, I know three such services: ustream, justin.tv and livestream.
What I like about ustream is that you can easily use a pop-up to view the
channel and use a real IRC application to chat. Perfect for watching. The
ads in ustream are annoying (as are all ads) but so far they are still tolerable.
I also like that ustream producer is based on a useful streaming application (WireCast)
and offers enough features to get a nice stream up and running.
Why aren't more people using ustream? Simple: most people who want to stream
have no experiences in video production and also next to none in using an IRC
client program. So they rely on the web-interface which is just a flash-bomb
as it seems. Also the flash app IRC client now lists new posts on top - to match
with Twitter it seems - which makes it totally unusable for anybody who is used
to instant messangers.
Justin fails for me... again and again. The commercials are played as video
overlays - which is annoying to begin with and gets me to blacklist any product
advertised - but the ad overlay just ignores the volume setting... waking up the
house if you're watching a stream at night. And to top it, most of the ads just
stall and block the view of the real video.
From a viewer perspective my biggest quarrel with livestream is that they
only have a crummy chat-like flash app... which is a pain in the behind to
use: wheel scroll doesn't work right, there's no "/me", and it's just laaagtastic.
Oh, and sometimes the player just fails to even start for me.
I still prefer to use ustream to stream and watch. They have not yet managed to
make their service totally useless in my opinion as have the other two. LiveStream
is at least tolerable when I only want to watch - but interaction via the chat
is impossible to enjoy.