Hi.
I have one. it works for me, in that I always get down in jan/feb - tearful, miserable, etc. and it does make me feel less down and sluggish at that time.
I have one which cost £200 as it's big and powerful, which means you are supposed to use it for 1/2 hour a day. Before that I borrowed one from a friend, a small "portable" one which cost about £100 but you have to use it for 2 hours a day to get the recommended benefit.
My friend, and I, both found the 2 hours a day hard to keep up especially with small children around. it's ok if you have a job where you can use it while sitting at a computer in a private office. I find my bigger one with the half hour exposure far more possible to use every day. (also, you should use them morning rather than evening to combat depression, so evenings on Mumsnet are the wrong time really to use them.
they do take a bit of getting used to, it's like your eyes going out into a tropical beach from a UK winter! there are lots for sale on the web. you can rent them before buying. Also people sell them secondhand as they don't suit everyone.
Mine is from the SAD lightbox co ltd. 01494 484851 but it has a website too.