My wife got me one recently (yay leftover Christmas gift cards!). I like it. I don't know how economical it is compared to buying soda, or just buying tonic/sparkling water and then buying the SodaStream syrups, but it is very convenient. I can fizz up OJ or grape juice if I want to (note: always use water in the machine as per the instructions, never juice - add the fizzy water to the juice or syrup afterward, trust me haha) and make whatever flavor soda I want. But you still have to buy the syrups anyway, so it seems like you might as well just buy a few bottles of sparkling water or just buy the soda and be done with it. I'm not a heavy soda drinker, so for me it's more about the cool factor & the convenience. Plus it was free, so what the heck. I'd been wanting one for years just for kicks. I use it now & then. It's cool. Eh.
FYI, Best Buy carries a ton of flavors (random place to buy it, I know, but my local supermarket doesn't sell the SodaStream syrups).