I enjoyed both movies. The kids in the first movie bring it down, but Ethan Hawke, Lena Headey and the storyline bring it back up. Also add a plus if you enjoy school girl outfits. It's very self contained. Giving you only the slightest look into the Purge world.
The second one dives deeper into the world including stupid middle class people who wait until the last minute to get anywhere on the day of the Purge. Different economic groups mix and purge or don't purge for different, often personal reasons. Frank Grillo is awesome. The youngest female character is annoying as fuck. Again. Mixed bag since they are both back in the third movie though she's probably more of a plot device. Hopefully.
There's an interesting theory presented, but a lot of holes. The holes make good fodder for discussing later. During the movie, it's basically a zombie movie. You're never truly safe. Your allies may turn on you at a moments notice. Everyone else is assumed to be an enemy on principle. Surviving and morals don't always go hand in hand. If you can just make it to morning or to a specific location, you might make it.