I have used it both before and after a Delta flight. I access the club with my Platinum (non-Delta) Amex and it is free for cardholders.

Overall, the club is decent. The advantages of the Airspace Lounge is that I have never seen it crowded and you get one free free food item (think sandwich or salad) or alcoholic drink with entry. They are happy to pack your food to go and include napkins and bottled water if you are running off to a flight, which is nice. The wi-fi is free and works reasonably well.

As for disadvantages, the club is small and I think they stream the TV channels for their TVs over the internet because they are really choppy. The club also doesn't seem that relaxing to me (maybe because of the size).

It's about a 4-5 minute walk from any DL gate in Concourse D. I wouldn't pay the $17.50 they are charging for it, but if you have an Amex that will get you in for free, it isn't a bad place to go.