I still use the cardboard boxes but I strengthened the bottom seams with packing tape. When I consider the upgrades I would like for home brewing I can't justify buying something that I can get free. My source for boxes is a nearby bar that goes through seven or eight cases of bud, bud light, miller lite every Friday and Saturday. If I need boxes I just swing by early the morning after and ask the guy cleaning up if he will let me have the empty boxes. He has always been very happy since it saves him breaking them down and hauling to the recycling center.
I'm only talking about six or eight cases of bottles in my entire inventory. Most of my turnover is kegged, which makes me happy. It is easy to add strength to the bottom of used cardboard beer boxes with a little packaging tape.