I've been trying to be cautious and not adopt early. I am an Enterprise Gold subscriber. I have app clusters for each of Python/Django, Ruby on Rails, and PHP (mostly WordPress). (edit: All servers are Linux.) I'm now considering taking the plunge. I have hosts to spare, so it's not the logistics of the conversion that I fear. (Unless you have a 5.0 Master + 5.1 Slave horror story to share.) I was reluctant due to a rant by Monty, but I really want a few of the new features. (Especially the additions to the information_schema.) I'm a full time DBA, so I like knowing what is happening to my servers and my data. I work for a news organization, so we don't have the freedom to experience down time like your average LAMP user. It feels like the time for 5.1 has come.
What say you?
If you have a compelling reason for doing so, go ahead when you get the mandate to spend the (large amount of) time. If I was your boss I'd like to know a really good reason for taking a lot of time and risk.
Upgrading a component like MySQL is going to be a massive piece of work, especially performance testing.
In any case, it really depends how well your application is covered by automated test cases, and how complex your release process is. But upgrading a database sounds like the kind of thing that requires every test possible.
You would at a minimum want to performance test critical parts of your application and do some soak / stress testing also to make sure you're not using it in some way which will blow the machine up over time.
Depending on how much data you have (and what the redundancy / downtime requirements of your app are; how big your maintenance windows can be etc), the logistics of the conversion may or may not be an issue.
I'd say that testing is much harder than logistics of a migration, even if the logistics are nontrivial. I recently upgraded from 4.1 to 5, and the project took ~ 6 months, mostly because the testing was hard. We found almost no bugs caused by the MySQL upgrade though.