Oh, if you want to hear what they sound like, try simplynoise.com on a low setting. I prefer the brown/red setting, which is quite similar to the noise the machines make (at least on the setting I use).

I own two of them, one for the bedroom, and one for the office or living room. I sleep much better with the noise machine on, and I find it easier to sustain high concentration for extended periods during the day with the noise machine on. The many little sounds that I hear living in a city and that I often find distracting when I'm trying to focus on something difficult bother me much less when the noise machine is going.