The ASR voice recorder app gets this right. It lets me use the full featured version for three days, after which I need to watch a few ads to get another three days. I choose when to watch the ads, and if I'm late there is nothing worse than a small nag at the bottom of the app. I actually now start every day with the ads, while I cook breakfast, and it is a positive experience. I could also just pay for the app and be done with them.