Well, in my case, it was LONG overdue.
I was sticking to v10 simply because it was reliable and I was lazy. There's so many fun things to do during weekends, upgrading an e-mail server not being one of them.
So, I did the upgrade from 10 to 11.1 and I expected the worse (it does look problematic when you browse the forums).
Actually, it was more of a migration. I moved the complete message store from the old server running SLES 9 to the new one running SLES 10. Then I did the install.
I did have some little issues, but none that couldn't be solved with some searching and some good ole RTFM. It took me a while to get my spam filter working, but I eventually figured it out. Same thing with the disk snapshots (that turned out to be an SLES 10 issue).
At first, SWA seemed kind of slow. Then, I reindexed everything. The next day, after all that processing was done, it was fine. I do have a server with quite a bit of horsepower (5Gb ram, dual processors, fast IO), so while I find the performance fine, I expected more.
It's been only 2 days but I have yet to hear a complain.
I have close to 100 users spread around 3 locations.
So yes, so far I'm happy with the upgrade.