At first I was trying to place the ads using code, but on Twenty Twelve, I could only find one code setup that would not throw a parsing error; but I could not get it to work correctly with more than one ad.
The user interface of this plugin seems to give great fine tuning of the placement of ads (maybe even overwhelming for some users).
I gave this plugin 4 stars just for the fact that it even allows for the placement of ads between posts, but as far as the control of that placement, it did not meet my expectations.
In the random setting, the (Google Adsense) ads are always displayed 3 in a row, and always along with the first or second three posts on a page. Trying to get more variety by using a mixture of before, random, and below settings still grouped all the ads together, and looked quite messy.
Although it's not perfect, I am still happy to have a solution for placing ads between to posts, and the random setting really works out pretty well for me.
Thanks to reviewmylife for developing this plugin.