specific issues and use cases for Magento Compilation module

I've heard various negative rumblings about Magento's compilation module causing issues. Is it now stable enough (e.g. v1.4.2) to consider using?

Can you describe the issues that a site might encounter when using it? I would guess that deployment of new code would be somewhat more onerous. What programming and administration practices are required to use it safely?

What quantifiable performance improvement should be expected? Does the site need to be serving a minimum threshold of traffic to warrant it's use?

For extra credit... The benefit in using the Compilation module is (AFAIK) in flattening the directory struct开发者_JS百科ure to the PHP scripts to be executed. If the site is already using APC or other opcode solutions, is the relevance of Compilation reduced?

Thanks,

Jonathan

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1. You definitely get the boost by easing autoloading the classes.
2. The errors are coming from 3rd party extensions who write stuff to their local directory or look for licence files from their paths and the path changes if compiled to src/include.
3. If you upgrade you always need to recompile but this can be automated