I always buy boxed for my own use at home, as I often just want a glass and no more, (which is impossible with a bottle!).
the decent wines are fine.....less decent aren't too bad, and the supermaerkets 'house red/huse white' are fairly good.....definitely good enough for the glass with dinner in any case (as long as your palate is not too used to the very decent wine)
money wise I find they are much better value, and it means that when friends come over, I don;t mind buying bottles as I class them as more of a treat then.
I do however like to use red wine in my cooking.......boxed wine is a must for then as it is less of a wrnech than opening up a bottle and not wanting red that night, and so possibly wasting a bottle.