I am using one of he cheaper ones since over a year now, and am quite happy with it. Of course, it still sucks there is a lack of real daylight in the morning, but the daylight clock does sort of pre-wakes me before my real alarm clock goes of.
Lately I have more trouble responding to it, but I guess that is because the days are ultra short at the moment, it is just very unnatural to be awake when dark. Also I don't have to get up to go to work or something, so I just set the clock and then wait to get up when the real daylight kicks in, which will be a little after 9 am...