United has not announced plans that I know of to add premium economy, but both United and AA nowadays tend to copy Delta, since Delta's been making a lot of money.

Of the big three legacy carriers, Delta is currently the best combination of price and reliability. You know they want a bit of extra money for the slightly nicer experiences, and you know what you're going to get out of it.

For international travel, though, I've been starting to avoid US carriers entirely in favor of flying the other country's airline (which is something you can do when living near a major international hub -- SFO, in my case). Often it's another small step up, and sometimes it's a big step up, compared to the US airlines.