People are often scared about freshness of the patches without getting into specifics. A lot of these performance optimization changes are rather local and so safe. It is like ant and elephant in comparison if you look at MySQL 5.1 or MySQL 5.4 changes. XtraDB is a more tricky beast mainly because it is based on Innodb plugin which is in active development itself.

I’ve also seen people willing to use XtraDB.

I’m now seeing people willing to try out 5.4.

5.4 and XtraDB are both alpha products.

I don’t see people who would be on the cutting edge with 5.4 or XtraDB, but are currently on 5.0. Especially since for many, 5.1 has given a performance improvement. So in addition to not seeing a desire (other than the one commenter above — nobody I’ve talked to has expressed this as an interest), I can’t imagine someone would want alpha patches on 5.0….anyone willing to take that risk should be willing to risk trying 5.1.

“may break” is experience based. All optimizer changes I’ve seen over last 10 years broke something. They typically fixed more things than they broke but it is not good enough for applications already in production – they typically have worked around things and tuned them for old optimizer behavior so new behavior may be unwanted. At least it should not be treated lightly and requires good testing on upgrade.

Some old regressions in 6.0 (I’m not sure how many of them are fixed now) can be seen here:
http://www.mysqlperformanceblog.com/2008/04/10/tpc-h-run-on-mysql-51-and-60/