So far it seems to be a very worthwhile move! I had looked into using ruby 1.9, but ran into may issues with gems.
Here are some Munin charts.
Mysql... this I don't even understand! What a difference!

Memory - also seems to be a nice drop.

Individual Interupts

Interupts

The webapp itself was not changed at all. it is still using Rails 2.2.2.