We were using vbulletin 4 for our forum about photography (http://www.ojodigital.com).
The system was being degraded as it is 17 years old and has tons of messages.
Vbulletin 4 is obsolete and the new version 5 did not meet our expectation.
I met discourse in the net by casualty, as I registered in a forum about router firmware that used it, and I liked it.
I showed it to the other admins and created a test site.
At first they found it very strange, but soon they began to love it.
In august we launched the new forum based on discourse ((https://foro.ojodigital.com) only accesible to selected users.
In september we opened it to everybody.
And they are loving it. There is much mor activity now than in the lasts months.
We are very happy with our decission and with discourse.