Edit: I got so excited I even forgot to thank Mitchell for all the hard work. Vagrant completely changed my (work) life. Thank you a thousand times, Mitchell!
Another benefit Vagrant Share has that I haven't seen yet in any other localhost tunneling solution is that it shares _every_ port (if you want), not just a single port, and gives the other person an IP to access the machine as if it were on the LAN. This is super nice if you want to expose anything like Redis, SSH, etc. You just "vagrant share" and, again, you're done.
Also, many localhost tunneling solutions don't support UDP. We fully support UDP connections in addition to TCP.
The big win, though, technology aside, is how if you use Vagrant, this just becomes another part of your Vagrant workflow that _just works_. You don't have to worry about installing anything, and of course it'll work with any sort of guest machine running on any provider.
Plus if you are at the point of QA/PM review of what you've done, the code has already been checked into source control. They could just as easily spin up their own VM with the Vagrantfile.
I used Vagrant for CI builds for a while, and I would have liked the ability to automatically share an internal url to the environment so that when coworkers submitted code that broke the build, they would have a single link to follow to get access to the particular environment on the particular server that serviced their job.
Some vagrant envs are also somewhat complex to set up - I could use this for teaching people how to use things like this: https://github.com/stackforge/puppet_openstack_builder
Anecdotally, it also seems to correlate with how useful Developers usually find their peers.
Also, as noted, seems great to let your colleagues see what's going on with your work.
This is already easily done just by configuring port forwarding from your host to your guest in your vagrantfile.
e.g.:
config.vm.network :forwarded_port, host: 8080, guest: 80
So, as you can see, if vagrant share works as announced it would make it much easier for me for that particular task.