Even though iMessage is far better at sending media than MMs, I wouldn't send any video that's more than a very short duration. It much easier to share video via Cloud solutions like Apple Photos, Google Photos or Flickr and much less stressful on your own bandwidth. Sharing photos though is terrific via iMessage..of course limited to other Apple users.
I have serious doubts that its adoption will be better than any of Google's other messaging solutions but have my fingers crossed that Allo does take off...need a great cross platform solution (sorry, Whatsapp is next to useless in the US).