Wouldn't it be better to use the email users feature in the ACP? At least this way you know that users will get the email message (assuming their email address is up-to-date of course). Sending a PM means that you can only guarantee that those who visit the site will see it, as not everyone will have email on conversation enabled.
Thats exactly what I intend to do... any suggestions?
I left IP.Board and found there are quiet a few features missing from Xenforo such as a good Gallery, Mass PM and Calendar.
Overall it is much better forum.
There are two good galleries for XenForo - Xen Media Gallery and Xengallery. Both written and supported by good developers. I use Xen Media Gallery as I prefer this and the support is excellent. Do check out both as it'll come down to personal preference.