My wife took a United flight from San Francisco to London a few weeks ago. This was the (somewhat) new 300ER with 10 abreast seating in economy (meaning narrower seats than previous 777s with 9 abreast seating). We were fortunate to grab one of the few two seaters at the back of the plane so it'd just be the two of us sitting together, making the journey a little more bearable.
Unlike most United flights I've taken recently, they actually had a choice of three entrees, one of them vegetarian. However, because we were at the back of the plane, invariably, the choices ran out by the time the food got to us (happened every time during the flight). So we were at the mercy of whatever was left. My wife is super organized so anticipating lackluster food, she packed some sandwiches for us which helped a lot. We did not order "muslim meals" so can't comment on their quality in United.
They had a decent selection of movies and TV shows on VoD. Their music selection was lacking but most disappointing was that they didn't have audio from the flight deck active so we couldn't listen in to the cockpit or air traffic control - one of the unique things United used to offer (at least for us geeks).
Furthermore, United doesn't offer a feed from any camera on the plane (either forward or down) so figuring out sunset or dawn or sunrise is harder than it needs to be. Also, as expected, United doesn't show prayer times or direction of qibla.
The restrooms at the back of the plane (didn't try any of the others) were ok in size (most are pretty small anyway). They did have auto sensing taps (not the spring loaded ones which are very annoying) which makes making wudu easier. The sink was also a decent size. The restroom wasn't a pain to clean (but not easier than normal easier).
Being in the back of the plane has many disadvantages (mentioned above, also deplaning takes a long time) but one advantage is that it's close to the galley at the back of the plane. So, it's easier to gauge when it's a relatively quiet period in terms of service and can go and request a space for prayers. The galley is big enough to accommodate most directions. And there is always the space in front of the lavatory (in this plane, there is only one lavatory on each side, thankfully, there aren't ones facing each other).
The ground crew was decently helpful. We had a separate ticket from LHR to Lisbon on TAP, she did check our bags through but after a song and dance that it needs to be within 10 hours etc (it wasn't). The in flight crew was fairly diverse and competent. They were aware of muslim prayers and even helped lock the toilet door while so that folks wouldn't come by wanted to use the lavatory.