I second Handbrake. Results work great with many devices and software including Roku, Plex, Kindle and pretty much everything else.

I simply plug the Roku into the my minivan's entertainment system and then the kids can use the built-in media browser to pick from hundreds of movies on a 1 TB drive that plugs directly into the Roku. No more swapping DVD's and worrying about scratched disks or skipping in cheap DVD players. RF remote works anywhere in the car.

Last Christmas I got each of my kids a Kindle Fire. My thoughts: The bad: software is klunky and ungainly, especially compared to iPad. They take forever to charge. Getting your own movies off and on is a pain. By the time you create a second profile for your kid, like half of the storage is used by system software, so you can only put 2-3 movies on it, which is nowhere near sufficient for a long road trip. The good: Amazon Freetime Unlimited. If you have an internet connection, there's a ton of content there that's age appropriate and the kids love it. Mine are age 6 and 5 and there's lots of content for kids their age. (books, games and video). My girls share an account because they are close enough in age that they are interested in the same content and it saved me a few bucks every month. Be aware that the video available in Freetime is streaming only though, however games and books are downloaded and are playable offline. I'm just glad that they don't harrass me every 2 minutes for in-app purchases and new games like they do when they borrow the iPad. As long as you subscribe, all content is otherwise free, unlimited, and no in-app purchases. Hardware seems to be decent enough quality and battery life is decent.