I'm managing a medium to high traffic news website(~15k users/day) for about two years now.
I have installed and configured WP Subscribe Pro last year.
In just a couple of days running I have noticed a huge drop of bounce rate(~5%) and a smaller but significant raise of pages/session (~0.2+).
This might seem small amount but if you manage a high traffic website, a raise of 0.2 of pages/session is a big deal.
I wanted to make sure that this (raise of pages/session and drop of bounce rate) was due to WP Subscribe Pro plugin so I added a trigger in the code for Google Analytics ( check this topic ).
Google analytics confirmed that the extra traffic was triggered by the popup.
During the past year we optimized the settings of the plugin. Because we have a lot of returning users we chose to use the subscribe form just a few days per month and the rest to show the related posts popup.
To sum up
After a year using it the bounce rate is ~8% down and pages/session raised ~0.3 from where I started.
The plugin is easy to install and configure and doesn't bother you with unneeded, frequent updates. mythemeshop keep it light, simple and to the point.
As you can see from my link above the support is great IF you even need it.
I will update my subscription to both WP Subscribe Pro and WP Review Pro.
PS: I'm not an employee, relative or forced to leave this testimonial by mythemeshop in any way. I'm just a happy customer.