As a top-tier DL member I'll chime in here.
There are a couple of passenger profile factors at play: Domestic vs. International and Leisure/Out-of-Pocket vs. Subsidized/Employer-Paid.

If it's not out-of-pocket and domestic, then DL is probably the best program with the CRC and time-of-booking upgrades.
Even mostly LU'ser with CRC, Skymiles probably ranks above the bottom of the frequent flier barrel.

International is another (stinky) kettle of fish.
If a minimum upgrade fare of over $1,000 to Europe is OK, then, DL's for you! Even so, the capacity controls can be *so* strict that folks are talked into waitlisting at those fares, denied the upgrade because Simply Good Business says *NO* same day upgrades *regardless* of front-cabin capacity at boarding. It is my understanding that gate agents have [almost] no leeway in this matter, barring an actual oversold situation. They *will* fly out with BizElite seats empty and folks sitting in coach who paid double the minimum upgrade fare on other carriers.
Platinums do get international lounge access on all tickets (in addition to CRC), though from what I gather those facilities are more on a par with other alliances' mid-tier than top-tier.

To recap - if domestic K fares aren't an issue, DL is the best. However, for out-of-pocket international pax, it's a pretty raw deal compared with top tier benefits on other carriers.