I use GlusterFS with Dovecot and it works without issues. The GlusterFS team has made huge progress since 2.0 and with the new 3.0 version they have again proved that GlusterFS can get better.
You have kindly shared some details of your config before - care to update us on what you are using now, how much storage, how many deliveries/hour, IOPS, etc? Lots of stuff was quite hard work for you back with Glusterfs v2, what kind of stuff did you need to work around with v3? (I can't believe it worked out of the box!) Any notes for users with small office sized setups (ie 2 servers' ish).
I presume you use gentoo on your gluster machines? Do you run gluster only on the storage machines or do you virtualise and use the spare CPU to run other services? (given the price of electricity it seems a shame not to load servers up these days...)
Thanks
Ed W