My girlfriend's mom got a soda stream. I have a few issues with it.
1) I find the quality of the carbonation subpar to store bought carbonated water.
2) It's still nearly as expensive as buying carbonated water if you use the Sodastream CO2 cartridges.
That being said, 2) can be solved by getting a non proprietary adapter and filling up generic CO2 cartridges for a fraction of the price.
Personally, I like PC carbonated water. The flavours are great, and it's significantly cheaper than competitor brands (e.g. Perrier, Nestle). $3 for 12 cans. Grapefruit is my personal favorite. Unfortunately, it's not as readily available as their other flavours.