It seems that the real value for your Delta miles, when you reach 50K is for two tix in the USA, especially long distance trips. You don't even have to stay the usual Sat night. Consider the cost of BOS to LAX or some other similar trip. Travel to Europe is relatively cheap during most of the year, except, of course, during the peak summer period. Try getting a seat in the summer. The accumulation of miles is only one part of the FF game, getting the most value for those miles is the real challenge.
WNF