I've been following this project for a while now and I'm just about ready to pull the trigger and switch my 7-year old / ~350,000 post forum to XF.
My only major concerns are the image attachment and gallery systems I've had in place for a few years and I'd hope for a great replacement for one or both of these.
1) I have a simple modification called VB Image Hosting (http://www.vbulletin.org/forum/showthread.php?t=106127) that was really basic: users could click a button / text on the Quick reply box and upload images that were added to their gallery (started using this before the official VB gallery) and to their posts. They could go back and review all these images, delete or make private etc later without entering the threads and chasing them down. It would show the BBcode to post them. Here's a little pic showing the gallery listing...
I don't need to convert this over, but I'd hope there's something similar for XF. I saw the User Albums modification, but does it have an easy way to add those images to post replies? IE, open a window and click the thumbnail and it adds the BB Code. That's my main issue.
2) The VB4+ image attachment system is actually not too bad these days. See the pic of VB4 I attached using the lovely XF feature.
On the test version of XF I couldn't access an Attachment manager to see all previous attachments (on the front end as a user), again not allowing me to re-post old / common photos.
Either a replacement for #1 or #2 would be fine by me. I just don't see a way to browse previous attachments or manage the images I've uploaded. I think my users will be upset if I take away these types of features and they would need to re-upload some images again and again.