What Has Been Your Experience with Moving Back to Mainframe?

We are in the process of evaluating a switch to mainframe (z9, in this case) from a distributed architecture for a 3-tier java app. I am interested in your experiences in doing something like this.

I've read some of the whitepapers and promo from vendors, but I am interested in hearing more about how things have worked - or didn't - in real-life.

With that in mind, I would appreciate hearing about your experiences - good, bad, or indifferent. You can post anonymously here, or email me at evalmainframe@gmail.com.

Some of my questions are:

• Was the performance as good as promised?
  
• Were there quirks with using the ibm jdk compared to Sun's?
  
• Did you have problems with security folks in your organization since you were now consolidating the database and appservers on the same physical machine?
  
• What ended up taking longer time than expected? What were you disappointed in?
  
• Were the savings from such a consolidation warranted?
  
• Was it hard to overcome resistance to "back to mainframe" from the distributed-thinkers?

Thanks for the info!