like walls but way faster built...
I hate to point this out, but currently walls are built near-instantaneously. Fortifications are, too. The only deciding factor in work speed is the distance between the rock and the location, and how quickly an available mason can get there. They don't really add much in the way of blocking water, because why wouldn't you just build a wall since DF makes it so easy?
I don't see sandbags as being useful in Fortress mode, but that's not to say they're worthless. Of the uses listed, only two really stand out for me. One is the idea of specifying the height of the sandbags to allow water to flow over it only if it's a certain height - well, great, but why don't we do that with walls? The other is that walls and fortifications take a painfully long time to deconstruct, whereas sandbags would not.
Fortunately, sandbags could become very useful in the ways described either for adventurers, or when we're allowed to take an army off site and wage wars away from home. Bags of sand are easier to carry than boulders, and you're unlikely to bring masons in your army to construct defensive walls for you. For this to work, you need to allow military units to carry and place the sandbags - I don't know if you want to allow everyone to do it, or specify a support class that could perform this and other tasks for you on the battlefield.
Honestly, I was against this idea when I first read it because I didn't see the need for a variation on a wall or a fortification, but as I talked myself through it I came around. Consider me a supporter now.