We have one too. My kids love it and since I like sparkling water I use it regularly and just put the drops of lemon or lime flavoring in and take stuff to work.
I recommend it also although after the cost I am not so sure you save money necessarily but it definitely saves you having to carry the cases of Coke out of the store.
I imagine over a period of time it will pay off but I am with you about storage. I noticed the Home Shopping Network of all places has a huge selection of flavors, but Walmart seems to do pretty good.
I wonder if the major players like Coke and Pepsi will offer their concentrates for this product?....