Delta Skymiles is definitely the red-headed stepchild of the miles & points world. It's the program that everyone (except Rene @ Deltapoints!) loves to hate. Delta still has a hopelessly broken award search engine, they recently devalued their award chart,
and their new earning structure is really bad for frequent flyers who pay for travel. It appears that there's even more reason for Gary to call Delta miles Skypesos.
And yet, Delta is still my go to program. That's partially because I'm based in the South and have long been captive to Delta's Atlanta hub. It also helps that Delta is one of the better programs (in my experience) for shoulder-season transatlantic (TATL) business ("O class") class travel. But I have access to AA/US through MGM and UA through BHM, and I still value Skymiles above AAdvantage Miles, Dividend Miles, or Mileage Plus Miles. Why is that? Because I hate, hate, hate, paying out the nose for "free" tickets.
I'm currently trying to book some TATL award travel for the Christmas season. Both Delta and AA have business class availability, but AA's primary TATL partner is British Airways, and BA charges exorbitant taxes and fuel surcharges. A TATL roundtrip in O would run 100K miles plus nearly $1000. By contrast a similar Delta itinerary is pricing out at 125K miles plus less than $100 in taxes and fees. US used to be a decent option for TATL travel with Lufthansa, but now that they've left the Star Alliance and joined Oneworld, US partner award travel will be hit with the same fuel surcharges that plague AA.
The entire goal of my points and miles strategy is to avoid (or at least minimize) the need to pay out of pocket for travel. Paying 100K miles plus $1,000 might objectively be a really good deal for a ticket that could cost $6-8,000, but subjectively it's a terrible deal because I'd never pay $8K in the first place. Sure, I could pay taxes and fees with a travel cashback card (like the Arrival or Venture cards), but I'd rather use points from those programs to pay for hotels or other travel expenses.
I also end up using Delta a lot because their miles are pretty easy to get. Delta is a transfer partner with Amex Membership Rewards, and there are a lot of ways to earn MR points pretty cheaply. Amex also offers three different Delta Skymiles cards (though you can only have one at a time) and there are pretty good targeted signup offers available. I applied through www.deltaamexcard.com and received a signup offer of no annual fee for the first year, 50,000 Skymiles for spending $1,000 in the first 90 days, and $50 statement credit. That beats the current offer on Delta's website by 20,000 miles.