Delta is consistently rated as one of the top airlines in the US in terms of performance and satisfaction, and has recently been in the news for implementing some changes to its loyalty program. We'll go over the basics of the four credit cards Delta offers, and who they make sense for.
In ascending order, Delta and American Express offer an A tier no-annual-fee Delta Blue card, the B tier low-annual-fee Delta Gold card, the C tier mid-annual-fee Delta Platinum card (not to be confused with the American Express Platinum card), and the D tier high-fee Delta Reserve card. I'll refer to this simply by their color from here on out.
Starting with the Blue, there isn't much to say, because it doesn't offer much. There's no annual fee, which is nice, but also no benefits other than the basic facts that it earns you Delta miles with each purchase (double at restaurants and with Delta themselves) and there is no foreign transaction fee. Even then, you're probably better off with other cards in American Express's portfolio, like the Everyday card, or the Green/Gold/Platinum charge cards, if you can justify the annual fees. The best use of the Blue card is if you want to cancel one of the other Delta cards, but would rather keep the credit showing up on your report.
The Gold card is more of a standard airline card. It offers similar earning of 2x miles with Delta and restaurants, plus grocery stores. Other purchases earn 1 mile. The main benefits are akin to low-level elite status: free 1st checked bag and priority boarding. These benefits apply to up to 8 other people on the same reservation with you, so families without status making a single long trip per year could get solid value, effectively paying $99 per year to check as many as 9 bags round trip. There is also a $100 Delta credit if you spend $10,000 in a year, but there are better cards with more value if you're spending that amount. The purpose of getting this card would be as described in the family example: you don't fly Delta enough to have loyalty status with them, but you do use them at least once per year and typically check a bag, possibly multiple bags for a family.
The Platinum card is where things start to get interesting. Compared to the Gold, Delta's Platinum card steps up Delta earning to 3x points, and adds 3x points for hotel purchases. The critical added benefits are a pathway to earning status, and a companion ticket. The $250 is certainly high for an airline card that doesn't offer lounge access, but you can also think of it as pre-paying $151 extra to have all of the Gold card benefits, plus better earnings and one round-trip domestic main cabin economy fare paid for a companion (minus taxes, usually $5.60 per way). Most Delta fares are greater than this amount, so you can pretty easily come out ahead. For the status boost, until the new program rules take effect, you'll earn 10k Medallion Qualification Miles (one of the multiple status qualification metrics) for spending $25,000 on the card, up to twice per year. Under the new program rules, you'll more simply earn 1 Medallion Qualification Dollar (soon to be the only status qualification metric) for every $20 spent on the card. Spending the full $50,000 that would net you 20k MQMs under the old system would be 2,500 MQDs in the new system. For reference, the lowest status will take 6,000 MQDs. This card is pretty solid as a keeper card for those who fly Delta frequently enough to chase status and want to take a companion with at least once per year, or just those who fly it occasionally, but want the benefit of the companion fare alone.
Last comes the heavy-hitting D tier Delta Reserve card with its $550 annual fee, with some serious added benefits. First, the companion fare is good for any class of service, as long as the flight is still domestic and within the lower 48. Next, the status boost has significantly higher caps and faster earning, up to 60k MQMs for spending $120,000. Under the new program rules, it will earn 1 MQD for every $10 spent, twice as fast as the Platinum card. Reserve card holders have "upgrade priority," which supposedly means you are bumped to the front of the upgrade list for anyone with the same status. I've actually had this pay off twice with upgrades to main cabin extra, when I don't have any status at all, meaning I was the top-priority upgrade behind anyone on that particular flight who actually had status. The last Reserve-exclusive benefit is lounge access, which also has old and new program rules. The old program allowed Reserve cardmembers unlimited Delta Sky Club access when flying Delta, plus two guest passes per year, and unlimited Centurion Lounge access when flying Delta. When the new rules take effect in 2025, Reserve cardmembers will have 10 complimentary visits per year, unless they earn unlimited visits by spending $75,000 on the Reserve card. This is a solid card for the same reasons as the Platinum card, but with the additional lounge access and faster loyalty boosts. If you can spend $75,000 on this card per year and regularly fly Delta, it's kind of a no-brainer; you'll earn unlimited Sky Club access and 7,500 MQDs under the new loyalty rules, well on your way to high-tier status with Delta, plus have the companion fare from the card. If you don't spend that much, or don't want to put it on a Delta card with relatively low earnings, the value in this card would be for people who fly Delta occasionally, but not enough to earn status, and who can make use of the companion fare for a first class.
The scenario that would make the most sense for a lower spender is then flying Delta for three or four round trip itineraries per year, some with layovers, at least one of which is in First class with a companion. This scenario would have you paying $550 for the annual fee plus $100 for two guest lounge visits to go to a Sky Club 8-10 times yourself, with the guest's airfare covered by the annual fee. You'd have a higher chance of upgrades to boot. This is actually exactly how Kristyn and I are using my Delta Reserve card right now. I will fly Delta once or twice per year for work, and we will both fly with them once or twice for personal trips. What the Delta Reserve enables and encourages is for one of our personal trips to be booked in first class. For example, next year we'll be flying to New York City on Delta in First class to try their new domestic first class seats on the A321neo, so around $1,400 will cover both of our tickets for over 9 hours in first class.
There are some common scenarios for which the Delta family of cards do not make sense, however. Solo travelers who rarely fly Delta and rarely check bags will not be able to make use of any of the major benefits. Even if their goal is to accumulate Delta miles, a better strategy is probably to get one of the American Express Everyday cards or charge cards, which earn Membership Rewards points that can be transferred to Skymiles at a 1:1 ratio. Likewise, frequent fliers on Delta who are travelling solo will only get value from the lounge access and upgrade priority, and the lounge access is limited unless you are a high spender.
Delta and American Express also team up on business cards, which are largely similar to the personal Gold, Platinum, and Reserve cards. These primarily fit with spending and travel habits the same way the personal cards do.