There is a search box above your post. Go to it and type in "Delta Economy Comfort" and you will pull up extensive discussions about this that should answer any question you may have.
I flew Delta RIC-ATL-LHR and return in May. I booked EC for the trip ATL-LHR, because it was an overnight flight, and for me, it was worth it. And I'm short and don't drink other than wine when I fly. Not a huge amount of additional space, but that little bit of extra roominess was worth it for me. Did not fly in EC on the way back, as it was a daytime flight, and I was fine with that too.Edited: 11:27 am, October 05, 2012
How much extra are they asking?
It varies based on aircraft. There is a huge thread in the Delta forum over at flyertalk that outlines each aircraft configuration.
I was lucky enough to be given an EC seat on a CRJ-900 for free yesterday. I had been assigned a window and asked nicely at the gate if they had any aisle seats (my shoulders do not like CRJ window seats). It had maybe 2-3 extra inches of legroom. Otherwise it is the exact same seat. Mexico to US also doe not include free alcohol like US to Europe does. I'm not sure what the cost would have been, but it was nice for a 3.5 hour CRJ flight.
I called Delta recently and was quoted $99 for EC on a flight from Minneapolis to Amsterdam in late January. They quoted $79 for EC on a flight from London to Minneapolis in early February. I haven't flown in EC but did notice the legroom looks much better on several different aircraft I've been on since it was installed.
I paid an extra 80 dollars for economy comfort on a flight from Atlanta to Los Angeles. Personally, I didn't feel it was worth the extra charge. The seats were still about the same comfort level as economy. I still received only peanuts, and I still had to pay to use the TV/Video screens (which I found annoying since my economy flight from Los Angeles to Atlanta was free)
There may be a chance that you won't have someone seated next to you because of the additional price. But if a flight is already full, I don't see the upgrade worth the charge on Delta Airlines.
As for myself, I am a nervous flyer, and comfort and distractions is very important while flying. I expected more for the price.
Thanks for your response,