Barefoot is definitely the best way to go for starting out and if you really want to get a feel for the line. It is how I started but eventually as I started doing tricks I started wearing shoes. Given the option of shoes or no shoes for me now though, I would choose shoes. I wear them whenever I slackline, except on the beach. No one likes sand in their shoes.
As for shoes go, I would say my favorite slackline shoes have been my 5.10 Basic shoes. I have gotten them broken in and I can feel the line just fine and they aren't totally thrashed. I slacklined in Vans Slipon for a long time but by the time the really got broken it they were falling apart.
My next pair of shoes though are going to be the
5.10 Line Kings. I had a chance to wear a pair of them for 3 days at the Winter OR in Salt Lake City, UT back in January and they were just awesome. I was put off by the hightop at 1st but I quickly got over it and fell in love with the shoes.
Take home message would be just find what you like and go with it. I still like to slackline barefooted but I enjoy wearing shoes as well.