We have 2 iPad mini's that we use to watch movies. Most of the time we'll rent from the iTunes store. Many of the DVD's we purchase now also have digital download where we have access to them for viewing but, to date I haven't attempted to watch one of those while in flight.
Many of the flights we've been on lately have wifi. Service varies and I'm reasonably certain download times are reflective of how many passengers are utilizing the service at any one time. So far the only one I've really been disappointed with has be SWA's service provided by direct TV. Most of the pages I wanted to view wouldn't load or required me to attempt to access them multiple times. I won't pay for SWA's service again based on that experience.
Keeping that in mind, we download the movies we want to watch a couple of days in advance. There's pretty large files and can take quite a while to download. At home it's not such a big problem but, if we're at a resort where wifi speeds can be iffy, it can take a long time. I generally wait until I'm about ready to turn in for the night, then rent a movie and download it. Then it can take all the time it needs.
Even though I carry a laptop, we use our iPad mini's to watch movies. If the person in front of you reclines it can be a PIA to use a laptop.