From a saving plastic point of view it definitely is as a family of four we used to get through a couple of 2L plastic bottles a day.
If you are going to be using it a lot then it works out a lot cheaper to get an adapter so that you can connect it to a larger gas bottle.
The costs of the refills can add up if you're using it a lot. The standard one costs around £18 and will carbonate around 60 litres.
So what some people do is to get an adapter and then buy gas in larger containers ( a bit like the ones for CO2 that you get in pubs and restaurants but a bit smaller).
You will have to pay a deposit for the container but the actual gas will cost you the same and it will carbonate around 450 litres rather than just 60 with the Sodastream refill.
Homebrew and wine making shops are often good places to talk to about who are the best suppliers of CO2 in your area.
We get a CO2 tank that contains 3.15kg of gas for about £18 and that lasts us for about 3 or 4 months depending on the time of year. It's about 45cm (18 inches) high and just sits next to the sodastream machine. If we were buying the regular sodastream refills I reckon it would cost us about £140 over the same time.
But we do use it a lot and probably go through about 4L a day on average for a family of four.
So, yes, worth it but do think about getting an adapter and using CO2 from somewhere else