If they work properly they will let you gain + relation with each lord who attends and remains till it ends. If you stop by and speak witch each lord during the feast you get more + relation also.
If you have a wife, give the feasts a try, if you don't actually see the quest for the feast when you tell her to send invitations then you have the same issue I do.
The problem is, for me and many others the feasts don't work properly when I have my own kingdom.
Also seem to be a few food items missing from the purchase list of villages which makes it very hard to throw a very good feast.
As a work around I used tweak MB to adjust the relation affects of giving fiefs. If you do that I'd recommend keeping it set to normal accept when your actually handing out fiefs or the enemy lords will be super loyal to their kings due to this adjustment + feasts.