Having used it extensively) and having contributed GlusterFS patches to make it work as the rootfs for the Open Shared Root project:
I can tell you that the performance isn't as good as NFS when used for the same purpose, the network latency being the same in both cases.
Similar discrepancy is apparent when it is used for things like /home.
I'm sure somebody will claim (without fresh comparable benchmark figures) that the performance situation is substantially different than it was back in 2010, but for reference, you might want to look at the figures I produced back then:
Specifically for an idea of how much difference being in userspace it makes on the server side you can compare the performance of knfsd vs unfsd (more than double). Then look at the nosedive in performance when you use GlusterFS on both sides of the equation.
GlusterFS is great for large streaming accesses, but if you have an IOPS sensitive load (and virtually all loads are IOPS sensitive), the performance is going to suck pretty badly.