As someone who just made 2P and spent roughly half my time split between E- and E+ I've come to the conclusion that it is not worth it for the following reasons:

1) Basically no shot at ever getting upgraded to first. Especially for those of us stuck mostly on Ted.

2) Early boarding - Sure it feels good to board before everybody else but I've never had a problem finding bin space unless I'm one of the last 10 people to board.

3) Priority security line - Nice perk for those airports that have them, but it doesn't really apply to me.

4) E+ vs. E- This one I thought the hardest about, at 5'9" my knees easily clear the seat in front of me when seated in E-. My issue is more with shoulder room which is identical between the two. About the only thing I miss in E- is the room to open my laptop which is sorely lacking, not a big deal though. As a big E+ fan initially now I can't detect a difference in comfort between the two (at least for me).

So I've come to the conclusion that I won't spend more than a $30 premium to fly UA domestically. If I can requalify for 2P next year I will, but I'm not going to spend much extra money to do it.