...any global data is guilty until proven innocent(Martin Fowler)
If you want to be a good software designer, don't optimize code prematurely. [...] If you use the Singleton pattern out of habit, because it "makes your code more efficient", you're prematurely optimizing. You suffer from Singletonitis(Joshua Kerievsky)
I'm not afraid of a few globals. They provide the global context that everyone must understand. There shouldn't be many though. Too many would scare me.(Ward Cunningham)
The real problem with Singletons is that they give you such a good excuse not to think carefully about the appropriate visibility of an object.(Kent Beck)
I mean, it looks like we're not lacking literature on the subject, are we? I wonder why so many people are just too busy with their Singletonitis to consult it (Kent Beck gave a hint on this).