I just did a quick test using Mailgun and recorded the results [1].

The TLDWatch is that Google does give you back the open event and they also give you the email address of the open since this data is encoded in the URL.

The data that is not accurate is what you would expect: IP address, geo-location and user-agent string.

We'll hopefully write a more extensive blog post shortly.

[1] http://blog.mailgun.com/post/gmail-open-tracking-test-at-mai...