We've been running 1.1 on about half of our servers for about a week now. Ive mailed before that I was pleasantly surprised by its better use of resources. Here's a graph showing that fact. Server load in the last 10 days.
It may be good to list your hardware, user count, mailbox backend, and file system information, as I suppose that this kind of improvement is not universal.
This specific server is a dual core 2.8ghz xeon with hyperthreading running on FreeBSD 6.2-STABLE. We have over 1 million mailboxes, with about 75,000 daily active users. At peak maybe 20,000 concurrent, in a mix of webmail and direct imap. (no POP, thats handled by different software).
The backend is a NetAPP 6070 with about 170 harddisks. All mail and control files are on this netapp, but all indexes are on local disks, and we try and make sure customers end up on the same imap server every time. As soon as all servers are 1.1 I'll start experimenting with NFS indexes.
Cor