JohnP since you asked and as a disclaimer I dont work for the company and dont have any interest in it, I have the Soda Stream
You can get it at Walmart here in Canada or through alot of other stores like Bed Bath and Beyond in the US, and you can exchange the gas canister there too. The syrups/flavorings can be bought online or at the stores, too.
I was really skeptical about the machine before I got it, but I tell you, it holds the carbonation really, really well. I'd say that my fizzy water is actually fizzier than Perrier on a regular basis. Seriously. I dont have experience with others but I did look at some but I didn't like how they worked like the iSi Soda Siphon. That one works with small gas cartridges like a Whippit does for making whipped cream. Reviews of that one say that there isnt alot of carbonation and you keep having to shake the Siphon like you do a Whippit to get the carbonation. Like turn the Siphon over, shake it, charge a small drink, shake it, charge again and repeat a bunch of times for each drink. I wanted something bigger, and the Soda stream lets you carbonate and store a liter of water at a time. Plus I have 3 reusable bottles on the go so theres always something in the fridge.
As for home made syrups, I'm a gingerale addict, so I make ginger syrup by boiling ginger chunks in water, then adding splenda until its sweet enough. I add it to individual glasses as I make it. I havent tried making vanilla or anything exotic, but if you look at the website there are tons of flavors available if thats whats gonna float your boat.