It highly depends on the case. Normally with something like my FT01 I'd go without a window, since the metal shell itself is pleasing to the eye. However, I got an amazing deal on my FT01-BW, and now that I have it, I rather enjoy the window. It gets me enthusiastic about working on my computer, and it gives me an incentive to go the extra mile to make the cables / etc. look good.
It's also highly dependent on the setting the computer will be used in. In a game room, den, certain living rooms or home theater setups - absolutely. In an office or any other room of the house that's meant to look elegant or "mature," a case with a window would be out of place.