Just started to get them - sadly only one for each topic, which is annoying.
It means you are forced to return to the site to get further updates on the topic. I know that on VB forums, it is frustrating as when I am busy I prefer to get further updates from the topic and then when I am ready I can pop back and reply.
Otherwise you end up forgetting about the topic and not replying to it.
So perhaps give a choice or send all updates via email.
Not sure if it is possible to get the content emailed as well - could not see an option for this.