DL has always been good to me when it comes to handling delays. The last 3 occasions:
1) JFK - MAN Flight delays by 2 1/2 hours due to a fuel leak on aircraft. New aircraft used coming in from PDX. One hour turn-around on that plane. As many Medallions as possible upgraded - regardless of fare class - Coach not full. Biscuits, snacks and soft drinks laid out on table next to gate whilst the lucky pax who were upgraded went to lounge. Three weeks later with no prompting from me, an apology letter arrives with a $100 Transportation Order.
2) Tornadoes hit ATL. ASA in ROA reroutes me to CVG and on to BOS. They wrote in computer that should I miss 30 mins. connex in CVG, pls book in hotel. That happened, Comair was having probs that night too and I ended up at a nice hotel in CVG with $30 for meals and a phone card.
3) Thunderstorms in BOS. Was called the night before and protected on UA via IAD - cancelling DL reservation. Tipped up at airport next morning to be told that a flight to ATL would in fact be going out (in 15 mins.) and that my upgrade was intact. Fine by me ...

I've been really impressed with DL when it comes to handling delays whether it be from giving out free (alcoholic) beverages / headsets in Coach for a 45 mins. delay or the above.