Stop using W3 — Plugin mostly abandoned, no support

By and large this plugin tends to work on older php versions and older server configs, though with each core or PHP update it becomes more fraught with issues. I have spent many hours troubleshooting issues that end up being due to this plugin. After about 5 years of using this plugin on a number of sites, I’ve moved on to alternatives. There are a number of simple caching plugins but they all lack the level of control of W3.
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Update June 2016: After some investigation, it turns out this plugin has not received a meaningful update since August 2014. All the updates since then were simply updates to the readme file. Time to move on.
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Update Sept 2016: W3, still without much communication, a complete lack of support and unresponsive to most requests, bundled a security patch with all sorts of new features out of nowhere recently. Requires a huge update for some just to get a security fix. Very sketchy development practices here to say the least. Still not recommended for most users.
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Update Jan 2017: WP 4.7 has been released for a month now and using this plugin with it breaks your entire site. (Documented here in the 4.7 master list thread). Again, users are strongly advised to completely uninstall W3 and switch to a different caching solution/plugin ASAP.