From what I've heard regarding the 737-800's teething problems, they were pretty bad the first couple of years Delta had the -800 but are now more or less taken care of. There were problems with the CFM56-7s, avionics systems, and IFE systems that did definitely require a lot of attention. As far as I know at this point, however, Delta is pleased with the -800 from a performance and range standpoint. It more than meets the company's needs, and one of the key criteria was hot and high performance in some key Latin American markets, and hot and high is where the Airbus falters and the 738 shines. The Airbus isn't even in the same league.
Within Delta, there has always been a faction that has tried to convince themselves and others that Delta made an enormous mistake by going all Boeing. As far as most rational people can see, however, the only Boeing that has been even close to disappointing has been the 767-400. The 737, 757, 767-2 and 3, and 777 are solid, market leading aircraft. Delta did the right thing in going with Boeing and has no reason to be even remotely disappointed, and Delta's on-the-record statements prove this.
Delta has made enormous progress toward moving to a nice, new, streamlined fleet with just two suppliers: Boeing and Bombardier. I simply can't see why Delta would take one step forward only to take two steps backward.
It's a new day. Every moment matters. Now, more than ever.