We bought a Soda Stream around Christmas and it's been pretty nice! I have to admit I was skeptical at first but it's proven to be pretty handy. We've replaced the cartridge once so far. We've tried some syrups but I mostly prefer to use the thing just for making carbonated water, as I'm not that much into sweet drinks. I also really like it that you can decide how heavily carbonated you want it as I personally prefer lightly carbonated water.
@Harmonie - Yes it does! That's pretty much its only function, actually - you attach a bottle of water, press a button and ta da, instant sparkling water! It's up to you if you want to add something for flavour afterwards. Well, at least that's the model we have, I'm not sure if there are other, more sophisticated ones...? There appears to be many different kinds and the prices vary a lot, but I'm not sure what the main differences are. The size of the cartridges perhaps? Which would affect how often you need to replace it. Idk, maybe someone has more info!