I've done testing with my own emails with a link to my own server. The image is still only downloaded once you view the email. The only thing that is any different is that the request comes from google instead of the user's IP address. This prevents getting or reading cookie data during the image request, but does nothing to prevent image-based tracking of email opens. For image content on non-unique URLs this could mean better loading speeds, but won't do anything to make email load faster for unique images.

Nothing stops them from pre-caching these images in the future, but for now it isn't quite as catastrophic for the email marketers are some article suggest.

Tried Epic out for myself. Looks nifty, but clearly not polished yet. Biggest issue is that it still leaks all the data from the user-agent and plugins. Disabling the plugins helped, but I had an even more unique user-agent string than normal. Seems like this should be near the top of the list for a privacy browser, but they don't even mention it on their site, at least from a cursory browse. Tested at https://panopticlick.eff.org