You know I have actually been really disliking the lounges lately.
Delta is over crowded a lot of the times anyways, so you aren't getting away from the busy airport. The food isn't any good, and it is much more worth it to pay for food out in the airport then eat the free food available..and that is saying something.
The only benefit is if you visit a Delta lounge like MSP, ORD, etc. which includes free beer on tap and other drinks...cheap beer, but its beer!
The Best Delta Lounge is JFK (Terminal 4?) although I don't get to fly through it too often. Downside is JFK charges for a lot of the food options (although they give you way more options).
Overall downside with Delta though is if you are traveling with someone you have to go in the lounge without them, skip the lounge entirely, or they will need to pay to get in. You can no longer bring friends, which was fun on the last trip I went through in ORD.
As far as the Centurion Lounges, I got to use the Dallas location for the first time recently and I was not impressed. It was surprisingly dirty (literally bbq sauce and food on the table) so it was hard to find a clean place to sit, and the employees to greet you either had a bad day, or get a lot of people trying to sneak their way in, because they were pretty rude and suspicious of me on 2 occasions a week apart. Also they close early, and will try and kick you out even earlier. I believe I arrived the first time at 930, had a late flight, I found out they closed at 10, thinking I'd get at least a half hour to enjoy a meal or something...nope started kicking us out 10 minutes later.
The food was much better than Delta, pastas, salads, bbq, all sorts of stuff. The food was comparable or better than some of the food you can buy in the airport.
Overall though even maxing out the $200 credit buying giftcards, making the fee 250 a year, its still not really worth it to me. I am going to cancel after I get my gift cards in January.