While I’ve not yet tried their IAP free trial; in the past trying to use IAP to provide a trial of some sort was frowned upon. As far as Apple were concerned, the free product had to be a useful product in its own right (the definition of useful was undisclosed), you were also not allowed to promote ‘upgrades’ or have disabled features, nor were you allowed to have a time limited version. You could have an ‘Upgrade’ button, and when clicked it would tell people the reasons to why they should upgrade. But everything else had to be hidden until the user choose to upgrade.
We tried it twice and on both occasions it failed. In contrast (for us anyway) having a lite version that used the same mechanic, but when the user clicked on Upgrade they were taken to a separate product in the app store, worked far better than an upgrade via IAP.
For me personally; I don’t understand it, all I can conclude is that there is something psychological about buying a separate product and receiving a separate product, as opposed to ‘unlocking’ features already existing in the product you have.
We have a pretty aggressive plan this year to modernize all of our Photography products (death of 32-Bit and OpenGL will do that). So maybe we'll try this new FREE trial on the App Store, if anyone else has tried it, I would love to know how it worked out for them.