These are popular in Germany. The only thing I don't like about them is the plastic bottles. If you haven't done so yet, check out the penguin system which uses nicer glass bottles and looks slightly spiffier. Of course, we haven't gotten around to getting one yet...we currently buy a local carbonated water in glass deposit bottles by the case for less than 50 cents per bottle. It's supposedly bottled by nuns who give the proceeds to charity or something.
After going through a 12 liter case every 2 days during the heat wave, we bit the bullet and got the sodastream penguin, mainly because of the glass bottles. It tends to make bigger, rougher bubbles than the commercial "sanft" (i.e. soft) option we usually buy, but not having to carry cases back and forth from the little store across the street is a major improvement. Downside is we don't hear as much neighborhood gossip.