These days airlines are will charge you for just about anything (bathroom use, Ryanair, really?) and most of it I refuse just out of principal alone. I hate feeling like I'm being nickeled and dimed. But there is one upgrade that I will pay for and that's Delta Comfort+ or Economy Comfort as it was previously known. Comfort+ is similar to what other airlines consider premium economy but for a fraction of the cost. To be offered this upgrade, you must book a ticket in the main cabin. Once you get to seat selection you should be able to upgrade or you can upgrade after your ticket has been booked, provided there is still availability. If your timing is right, you can get an upgrade for as little as $49 on international flights but usually expect the upgrade to cost around $99. And for domestic flights, it can cost much less but will largely depend on the route. Now, the good stuff. Here is what you get when you upgrade to Comfort+:
Sky Priority boarding
Dedicated overhead bins
Extra legroom (3in. domestic; 4in. international)
50% more recline
Free booze and premium snacks*
Seat in the front of the economy class cabin
*Applies to domestic routes where booze, better snacks and entertainment costs you. On international flights these amenities are complimentary regardless of whether or not you upgrade.
So the big question, is it worth it? Personally and for the legroom alone on an international flight, absolutely. I am not a very big person but 6+ hours in a cramped space is painful and totally worth even paying $99. When you're already paying $1300+ for a ticket, what's another hundred or so when we're talking about being more comfortable? But for a domestic flight, it all depends on how much Delta is charging and how long the flight is. Anything over 4 hours is worth considering for me. However if you are tall or have some achy body parts, anything regardless of flight duration might be worth considering provided the price is right.
Lucky enough to have status with Delta? Silver Medallion members can upgrade to Comfort+ for free 24 hours prior to takeoff, Gold Medallion 72 hours prior to takeoff, and Platinum and Diamond Medallion members can upgrade for free at time of booking. Happy traveling!
More info on Delta's site here.
UPDATE: As of Nov. 17, 2015, Delta has made Comfort+ a booking option like Main Cabin or First/Business Class for U.S. and Canada flights departing on or after May 16, 2016. In other words, you no longer will choose Comfort+ on the seat selection page. This also means that Comfort+ will be booked through to your final destination rather than you having to book it separately for each leg.