My experience was not as bad as yours, but the problems I saw at a similarly small scale (6 servers, 3 2X replica sets) make me glad I'm not having to scale up a GlusterFS infrastructure. The biggest problem I saw was with absolutely atrocious performance when healing after a temporary node loss. Even if it was just a loss of a minute or two (server reboot), it wouldn't lose availability or corrupt files, but performance would get so bad for the next 2 hours that it might as well have been down.

If I had to make a comparison, I'd say GlusterFS reminds me a lot of MongoDB in the beginning. It wins a lot of kudos at the outset based on ease of setup, management and CLI UI, plus it has a good "story" on ability to scale up that gradually begins to fray when pushed. Hopefully there have been big improvements.