I built a Raspberry Pi cluster a while ago, and it's been running really well. I got my hands on 4 Banana Pis a month ago, and I thought I'd build another cluster. This time around I've used Nginx instead of Apache. I'm using a CMS I wrote myself called Pyplate. The latest version is much more efficient than the version I used in my RPi cluster.
The Banana Pi is a lot more powerful than a Raspberry Pi because it has a dual core SoC running at 1GHz, and it's great for web servers because it's got gigabit ethernet. The Banana Pi also has a SATA port, but I'm just using SD cards for simplicity.
Check out my site at Banoffee Pi Server.