If you use Marketo's send to a friend token, you can get information on who the email is forwarded to.
To find everyone whom an email was forwarded to, you can use the 'Received Forward to a Friend' filter. Similarly, you can see all the people who forwarded emails using the 'Sent Forward to a Friend Email'. The one drawback of this functionality, is after the first email is sent, the person is automatically unsubscribed. I typically recommend that people do an operational email asking if those people who received the forward to a friend would like to remain on the email list.
I personally have not use iHance but have heard from some of our clients that they have, and like it a lot. It's nice because the Sales people do not have to change their workflows whatsoever. This to me is the biggest advantage over MSI, as even with the Outlook plugin, they still need to change their workflow slightly.