We've had one for about a year now, and really like it,
We buy most of our flavors from Kohl's (using coupon) and Bed, Bath, & Beyond (using coupon)... then buy the CO2 cartridges from BB&B... have only had to buy 3-4 cartridges since we originally bought the machine.
When we first bought the machine, we bought four 1-liter bottles and four 0.5 liter bottles and haven't bought any bottles since... They hold the carbonation really well till it's gone, but that's usually by the next day if not sooner.
The flavors are decent, imo... don't really taste like brand names, but really not bad at all. They are only 35 calories per serving vs. 100 cal/serving for brand name regular drinks... but they definitely don't taste "diet" to me (I can't stand diet drinks).
The only ones we use are cola and pete's choice (my favorite), and then on occasion do the lemon-lime, orange, and rootbeer.
We do still buy Pepsi maybe once every couple months (just cause I have to have real Pepsi at times, lol).. but if I had to go without brand-names and just use the sodastream, I could.
It's really convenient, has saved us a lot of money... get the bottles for about $4, makes 400 oz's, where we normally could get a 6-pk of 20 oz Pepsi's (120 oz's) for approx. $3.67 (three 6-pks for $11).
Eventually I'd like to try the flavor essence flavors, but haven't gotten around to it yet.