Did anyone else feel 'The Entire History of You' was a complete waste?

Your post actually really does put it in perspective, thanks, moviemasochism. The episode is actually wiser to me now that you've put it that way...and you're right, we essentially have access to learn semesters of knowledge which was once confined mostly to universities. But instead, it's easier (and preferable) for most people to watch funny cat videos and post semi-clever memes on social media and argue with idiots. So much for human progress :(

Other repliers kinda hinted at this, but I think the episode was saying that this piece of technology, which initially looks awesome (I can relive myself having sex! I can sit and watch the best day of my life whenever I want!) would ultimately just make us bitter and paranoid and unable to really live properly.

We already obsess over all the little details anyway. If you see your wife glance at another guy and wonder if it meant anything, it might haunt you for a while but you'll otherwise just get on with your life, maybe thinking about it from time to time. Unless...you have technology like this, in which case you can sit up all night with a bottle, watching it over and over again. And humans being humans, that's exactly what you'd do. We'd be an entire race of people living in the recent past, never the present, given carte blanche to over-analyse and over-analyse all the things that we already over-analyse now.

I personally really liked the episode, but in looking at why it might be considered a waste of time, I do think it's fairly atypical for the series. It tells a 'smaller' story than the others- it's set only over two days, like a 'snapshot' rather than a full, fleshed-out narrative, and we are given no indication of what life is like outside the bubble of this small circle of people. None of the characters are likable, we have no-one to really root for, and neither are we given a villain or a huge central conflict. It's not particularly funny, either; it doesn't really balance light moments with dark moments, it's just kinda the same tone all the way through, with a low hum of paranoia and awkwardness hanging underneath, nothing Earth-shattering. No twist, no nightmarish imagery, no killer robotic bees or man-sized 'roaches'. It should be a minor Black Mirror episode, but I don't think it is. Its technology is fascinating, and unusually, something I would consider having myself if the opportunity arose. I imagine many other people would too, whereas no-one in their right mind would choose to live the way they do in Fifteen Million Merits or Nosedive.

They don't just replay events in their mind, they can scour social media for any slightest hint that verifies their suspicions. "I watched you laugh at his lame joke over and over" is not so far removed from "I kept looking at how you liked his lame post over and over".

I think the reason they feel like half a story is that the episodes are modeled after short stories rather than novels like movies are.

A short story is a brief fictional prose narrative that is shorter than a novel and that usually deals with only a few characters. The short story is usually concerned with a single effect conveyed in only one or a few significant episodes or scenes. The form encourages economy of setting, concise narrative, and the omission of a complex plot; character is disclosed in action and dramatic encounter but is seldom fully developed.
(This is from Britannica.com)

That describes Black Mirror episodes pretty closely.