I can't speak to Delta's program, but I can to AA, which is probably reasonably similar.
I'm a lifetime platinum with
AA. What it means to me, on AA and
other OneWorld carriers:
- Check in Business Class line
- No baggage fees
- No fees for premium seating (legroom, location (exit))
- Faster security line in some airports
- Priority boarding
- Priority placement on flights
- Ahead of Silver/Gold in terms of upgrades, but behind Exec Plat and secret levels
- Better priority when calling reservation/assistance
- Better seat selection (not hidden) online
- Lounge access internationally (but I also pay for an Admirals club domestically)
- No fees to redeem miles
- lots of other hidden benefits - basically you are treated a bit better.
I got lifetime with 2M miles, I have over 3M now. But a fair hunk came from the years of triple miles and use of a mileage credit card (buying airplane tickets). The perks definitely influences my airline choice for mid or long-haul flights.