I built a Raspberry Pi cluster a few months ago, and it’s been very reliable. Performance has been surprisingly good, and the cluster has shown to handle a reasonable amount of traffic.
I recently got my hands on four Banana Pis, a Raspberry Pi clone with a dual core processor and gigabit ethernet. I’ve built another cluster and got it on line at BanoffeePiServer.com. This cluster has half the number of nodes in the RPi cluster, but it’s much more powerful.
I’ve used a newer version of Pyplate CMS which is much more efficient, and I’ve used a much more efficient mechanism to synchronize the servers. Pyplate uses an SQLite database, so there’s no need for database replication. The entire CMS can be copied to each node with rsync.
This page explains how I built the Banana Pi cluster.