I've flown United, and it's not that bad. I flew to London last year (2011) booked through QANTAS with codesharing QANTAS, Cathay, Air France and British Airways, flights, and I found the Air France leg the most comfortable, and compared to United in 2010 and 2009, other than the in-seat entertainment, it was about the same as Cathay comfort-wise for the long haul flights. ****SHRUG **** heck, I was almost contemplating flying United to London, it was only going to be an extra one or 2 hours going via LA, but everyone always says United is so bad, I thought I should try QANTAS internationally for the first time in ages, and ended up trying 4 different airlines.
The internal flights was a little bit more of a step down, I was on one flight from Chicago to Miami and they didn't even have the drop down TVs from the ceiling, no entertainment anywhere, but that was only fairly short flights. The cabin crew are generally more on the mature side rather than bright young things, particularly on internal flights, no glamour in their staff. Since the economic crisis, places you'd expect younger people staffing it are suddenly full of older people, eg DisneyWorld used to be all youngsters as CMs, or at least the CMs that dealt with the public; my last 2 trips, there were a LOT more old people as CMs, ones that looked like they should be retired but they lost too much money and now they had to put on a Disney uniform to get a job, and that applied pretty much everywhere I looked closely, any low paying job that kids used to get to get them through uni or whatever seemed to have been taken over by their grandparents. Don't know where all the kids now had part time jobs - maybe they were spending their grandparent's savings.
So United isn't swish, but it is OK, if you can keep everyone entertained.
I will say a couple of major advantages with United is that you can get all the internal flights with them and that can make things a lot cheaper and a lot more manageable if there are any connection issues, and it means absolutely no concerns about paying for luggage because you have an overseas ticket.