Here's my two cents on it:

I've had DM for a couple of years now. It was originally just a "bucket list" thing that I never expected to achieve as normally I would earn enough for PM and not much more. A few years back I found myself with DM in range because of all the Covid bonuses. Combine that with the ending of MQMs and status rollover, I've had DM ever since. This upcoming year (2026) is my last year of rollover status, so 2026 will be a year to see where my earnings end me for 2027. For comparison purposes this year I will end with $27K+ MQDs, just shy of the $28K threshold to make DM organically. There is a possibility some upcoming expenses on my DL Plat Amex could push me over the 28K mark, but I'm kind of hoping they will not be incurred until 2026 to count for 2027. But it's not expenses that are really within my control, so whenever they happen will happen.

As for DM being worth it? Or useless? Well, I'm divided on that:

As far as upgrades go? Not worth it. At least not on the routes I fly. There's usually 40+ people on the list for 0 or maybe 1 seat. Living in ANC flights are almost always full of paid F. And because of that, I myself generally buy paid F these days. I fly back and forth between AK and FL 3 or 4 times a year, and barring a family emergency, I have no interest in sitting in the back for trips that long. I buy F on DL (or occasionally AS or AA) and don't play the upgrade game.

RUCs/GUCs: Partially worth it. I've given up on RUCs because I don't want to play the wonky inventory games DL requires, as well as the IT headache of getting them applied correctly. GUCs fall into the same category for international D1 - you'll spend hours playing inventory games to actually use them. (And I say this because all of my trips involve connections, not non-stops). The sweet spot for me has been GUCs on domestic F routes or international routes where no D1 is offered (like BOG). Those are last seat availability so easy to confirm on all segments. That's one case where having DM is worth it to me.

Phone service: Worth it. You can get connected to agents much faster. Now the quality of the agent is always a concern, but you can always HUCA if needed.

Extra earnings on tickets: A nice perk, but not necessarily something to push it into the worth it category for me.

Better customer service: Eh, I can't tell the difference. I get thanked for my status on maybe 50% of interactions with DL employees. Most of that just feels like something they are required to say. Only occasionally do I feel it''s genuine (it is a business after all).

C+ Upgrades at booking: Worth it, but honestly PM gets the same. My husband is also DM and will be falling back to PM next year. This was where he drew the line for chasing status. He's found that he likes this benefit, and will work each year to obtain it by earning at least PM. For me always planning to buy F it kind of negates the purpose.

Increase in baggage allowance is also nice, but can be achieved with lower statuses as well.

Overall I'd say DM is something "nice to have" but it's not a "must have" in my book. Of course everyone's outlook is different and different folks prioritize different things.