On-board wifi beyond the aircraft does not have enough to handle streaming from Netflix or HBO GO. It's even spotty for reading email or browsing the web. However as pointed out by Silver Fox, UA does have their own streaming which uses the UA app on iOS or Android on some flights. I've used the UA app for streaming and it works well It doesn't work well when streaming to a laptop browser.

You also have the option of downloading a few movies onto your tablet or phone before you leave. I've done that to my iPad for my kids and that also works well since no on-board wifi is needed.

Also you can find details on your aircraft from seatguru.com. Their data are usually pretty accurate.

nrg