Maybe I've just been lucky but I've never had to pay anything extra to get a seat in United's Economy Plus section. My loyalties are to United and I try to fly them whenever it is feasible. When I check in, I'll ask if there is a seat in Economy Plus and 9 out of 10 times there is. I'm not charged any difference for doing so.
On a recent flight from Las Vegas to Denver however, I was told that there were no Economy Plus seats available as the flight was full. It wasn't a problem and I just sat in the terminal area to await the flight. Shortly after, the United Agent called me to the desk and very kindly said that she was able to find a seat up front for me.
What makes this special is that I never gave her my name when I originally requested the seat. In order to do this, she had to look into her computer and try to figure out who I was.
Now that's what I call service.
I don't believe the standard economy seats on United are any different from those that you will see on AA, Delta or the other carriers. AA did have additional seat pitch on some of their aircraft but as mentioned earlier, they are doing away with this and putting the seats back.
United's "Economy Plus" section has been around for years and it works for them.
United is a great airline to fly.