We (DGF sunset_princess and I) went last year and I can honestly say that it was one of my favorite restaurants at WDW!
It was a toss-up between 50's and Sci-Fi and, after looking at the menu (DisBoards won't let me put the link up but you can find it at allears.net), we went with 50's for dinner.
One of the first things that you notice walking into the restaurant is that the ambiance and decor grabs you immediately - you are, from the outset, plunged into 1950's Americana - from the carpets to the tables to the snippets of TV on the huge, multi-coloured CRT sets in all manner of art-deco stylings and the wonderful staff!
Being called 'hun' and told to make sure we ate all our 'greens' made us both chuckle, the style of food and the presentation was exactly as we had expected and getting told off for making a mess made me laugh right out loud! There are stories about people being told to stand up against the wall for putting their elbows on the table or not finishing their meal that made me cringe initially but it's all in good fun and if you check out the pictures of it happening to grown men and women while their kids watch, you'll see everyone laughing about it (apart from 'Mom - the waitress' who is doing her very best to keep in character!).
Obviously you don't just go to a restaurant for the setting; you go to eat! But, trust me, the food is also fantastic! I had a steak and DGF had the fried chicken. We also had onion rings to start, and... desert (I forget which!). Everything was brilliant; the steak was cooked perfectly, the onion rings were full rings - not the beat up onion bits in batter - and were crunchy and delicious, the chicken was AMAZING (I'll be having it when we go this year

) and the sides were great. The portions were spot-on and the milkshake that I had at the end of the meal was the best I'd had in WDW (vanilla FTW!)!
I'd recommend 50's to anyone and it's one of the places I'm looking forwards to eating in the most when we go in September!
What ever you do, though, I hope you have a great time!